Hi! Welcome to you and all of your loved ones!
I presume since he is a full-time user, he will be prepared to self-propel for (up to) 10 miles per day. Just as an FYI, be sure to let him know since he may want to bring some gloves (even inexpensive golf gloves from Walmart can help protect hands) or it may change his plans regarding the chair he brings.
He may want to bring his favorite backpack - that will allow him to keep a water bottle, and any personal care supplies he will need. Generally speaking, I still recommend that when it come to personal care and/or medical supplies, it's best to pack those into clear, zip top gallon storage bags - that way, if the fine folks at Security need to peek through the pack, they don't touch your "stuff". (This applies when flying too, BTW! TSA is full of fine people, but I don't want anyone touching my personal care stuff - gloved or not!)
I have to confess - Saratoga is still on my "bucket list" at the moment, so I have never stayed there. However, the DVC Resorts and the Deluxe Resorts at WDW tend to be pretty similar, and so it is to those similarities that I will speak! Hopefully we will have another member come along here who has recent, direct experience with the room type you are staying in.
Personally, when I travel, I take along 2 lengths of
this material to use on the shower floor, and a second one to use just outside the shower for my helper. It's just mostly for my peace of mind; I don't want anyone slipping! I also request extra towels from Housekeeping as soon as we arrive; just to make sure I have a couple of extra in case the shower runs out too far on the bathroom floor. They are always super accommodating about leaving extra towels - and I always tip daily to say "thanks"!
The beds *should* be at transfer height - although bringing along his transfer board is still a good idea. Generally speaking, there should be enough room for the chair on at least one side of the bed (in the center, if there are two beds) and the bathroom will be well-laid out. At SS, you should have a "true" roll-in shower; we had a surprise stay at Boulder Ridge Villas last January and found ourselves in a room with a "roll UP" shower - you had to roll up next to the bench and transfer in to the shower, because it was a very narrow stall with a fold-down bench. I had to transfer on my "bad" side, which made it really uncomfortable.
Otherwise in the bathroom, expect handrails at ADA heights, and there should be ample room to transfer properly to and from the toilet. The sinks can vary a bit - at BRV, for example, it was a pedestal type sink, but the height was fine, and there was not a far reach on that sink for the faucets. Because we were in a 1 bedroom Villa, the vanity sink was not a "roll up", but the kitchen sink was. The microwave in the kitchen was over the stove - so not useable by anyone in a chair, and the washer and dryer were stacked, so the dryer was not useable without an assistive device or helper to start it. Ironically, as we were checking out, one of the managers came by the room to check on us (we had to be moved due to an issue with our "regular" roll in shower room in the main WL building) and I took them on a quick tour of the room, and pointed out several things that were not really HA, and that could easily be improved.
This won't really apply to your situation - just general info
Some people have complained (historically) about the closets at SS in the HA rooms, feeling that they are not as accessible as they should be. Again I have no direct experience with that, but generally speaking, in other HA rooms I have stayed at within WDW, there have been some issues. I love - adore, really - POFQ for example, but the armoire that they provide in lieu of a closet in the roll-in shower rooms there is problematic at best for someone in a chair. We deal with that by me using the entire top drawer of the dresser (packing cubes for the win!) and half of the middle drawer LOL.
Definitely bring along ponchos (rain) and I always pop a couple of clean, cotton bandanas into one of my clear zip lock backs in my pack - they come in handy for *so* many things! Everything from mopping up rain, sweat or condensation to a quick headband/hairband, even looping shopping bag handles through, and making one big loop to carry everything easier! You can find them for a $1 each at your local Walmart, often in fun seasonal varieties.
Getting around WDW is super easy for your partner - all of the buses will be accessible. At the Resort, just have him wait in the white painted rectangle on the ground with the HA symbol
in white paint. As the buses approach, he can signal the driver that he is waiting for by nodding yes (I want to ride your bus) or no (I'm not in line for your bus). Once the bus stops, and everyone is off, the driver will typically close the doors, kneel the bus, and then open the back doors and flip out the ramp. Wait for the driver to motion your partner on, and then up the ramp he goes. The driver will indicate where to park, and will likely ask if your partner wants to remain in his chair; all he has to say is that he does not transfer. (BTW- that is the magic phrase to use everywhere at WDW - "I do not transfer" which informs Cast Members that he won't be moving out of his chair for anything) The driver will also usually ask how many total are in your partner's party.
The bus driver will then "tie down" the chair with your partner, by clipping on the tensioned belts down at the wheel level, front and back. He will also offer your partner a seat belt to help hold him securely in place as the bus moves through traffic. The driver will indicate to you when you can walk up the ramp to join your partner, and you should be able to sit near/next to him.
As far as boats go, *most* boats at WDW are quite accessible; the notable exceptions are some of the small launches that serve Poly/GF to MK and back, and a small launch that runs every other trip on the WL/MK route. These will be obvious to you - there will be a step down into the boat, and no accessible opening. All other boats will have a portable ramp that a CM (Cast Member) will place for your partner, including the Sassagoula fleet that serves parts of Saratoga, and the Disney Springs Water Taxis (which are the same type of boat. The Friendships that ply the waters between Hollywood Studios and the International Gateway at EPCOT use the exact same ramp as well.
The Monorails use the same portable ramp for loading/unloading as the boats use, and the worst part is some of the ramps going *up* to the stations may require a manual assist for your partner. A CM will place the ramp for your partner - just follow their instructions.
The Skyliners are VERY accessible, and the gondola cabins are a direct drive on, no ramp needed. CMs will "chock" the wheelchair's wheels to prevent it from rolling across the cabin while in flight. The user simply rolls straight in and sits between the benches. Follow the signs and CM directions at each Skyliner station to go to the HA loading area - they fully stop the gondolas to load there, and your partner can take all the time he needs.
Disney Springs is super accessible - he should have no problems there at all. There is one bridge that is a bit steep going over the "springs" that he may need a bit of an assist with, but otherwise it's pretty easy.
A note here about First Aid: There is a First Aid center in each of the 4 Parks, and they can help with a place to lie down if needed for a short time, generic Tylenol, band-aids and a quiet place to administer treatments/medications if needed. If your partner overheats, do not delay, do not pass go or collect $200 - go straight to First Aid. (Also remember that ice water is free at any Quick Service food location)
As far as "fun" goes, I have been unable to ride a lot of rides for years now at WDW; because of damage in my back, I can no longer ride any roller coaster, or anything that "throws" you around too much. Having said that, I don't really miss those kinds of rides - I find that there is SO much else to do and see at WDW, and I typically will run through the queue with my family members, and then I go on out through the "chicken" exit, and do some grade-A people watching, or go get some ice water, or maybe a bit of light shopping :::ahem::: while they ride... whatever LOL. I'm not really a thrill ride person to begin with, but I love love love things like all of the little museums tucked into corners of EPCOT World Showcase (like Japan, Norway, Mexico, etc.) and I actually love the shows and attractions that others view as a way to cool off and nap!
Disney World is so visually rich - and the fact that there is a "soundtrack" to go with every place you are just adds to the experience - that I don't mind waiting like I might at other places. I often use that time to search for last minute FP+ or ADRs in the My Disney Experience app, or I might just pull up a Park map and look to see what's around that we haven't explored recently. Personally, I have no regrets with any trip I have ever made - if I could, I would relocate to Florida to be closer, and I would buy an Annual Pass before I bought my house!
Worry less about what he *can't* do - and just enjoy being at Disney World together! You can always find Magic there - sometimes it's just in different places, and different ways of doing things!
Oh - and don't forget to stop at Guest Relations on your first Park day, and get him his "First Trip!" button with his name on it!
I‘m not sure she believed me when I told her this place was a wealth of information and kindness. 
Our whole family is so looking forward to taking him on his 1st Disney trip.
