As a teenager, I didn't mind Christmas at MK at all, especially when we brought friends. We would all just hang out, people watch, and talk in the 2 hour lines. Bodies literally pressed together in the hub was annoying and made it hard to stay together, and you didn't move anywhere fast (I don't know how people do it with strollers or wheelchairs), but if you are just looking for some fun and Christmas cheer, and don't mind a mosh pit every once in a while, it can be a blast. My local amusement park can see long lines as well, and doesn't have anything as efficient as FP+, so it's all in the expectations and the kind of experience you want/ your son can handle. The biggest difference to me is that the walkways are much more crowded than slower times.
That said, if you stay on property and hit the EMH early (like in line 30 minutes before opening), then 7-11 at Christmas is about the same as 9-1 during a moderate week. Touringplans tells me you can ride the same number of attractions in both those times
So I would sacrifice morning sleep and follow the advice of people here and get to the parks early and get out - eat an ADR at 11 or 11:30 if you really want to eat at a restaurant in the park, otherwise, I would just as soon eat at a resort restaurant. You will be hungry by then because you got up and ate breakfast so early! Evenings will be crowded, especially around parade and firework routes, but you can make it fun. Maybe just plan for mornings and let afternoons be chill, and then take evenings as they come unless you have ADRs or dessert party tickets. Most of all, be ok with leaving a crowded park and doing something else. The resorts aren't very crowded at all. You can go to Animal Kingdom Lodge and do some of the fun animal activities or watch the geyser at the Wilderness Lodge. Go mini golfing. If you don't mind paying for activities, then there are fun tea parties, pirate adventures, etc. that are great fun and capacity controlled. You could also have your sensory son go to your resort's child care center for a night (they do fun things for them), and let the rest of the family enjoy a crowded park if the crowds are too much for him and everyone else really wants to go one evening.
If your child is anywhere near being able to use a stroller, do it! My 8 year old would happily ride all day if I let him, so definitely consider the option, even for an older child. A lot of strollers have carry capacities as high as 100 pounds. For a teenager, just try to be as engaging as a friend and upbeat and not stressed about the crowds so he gets the body and speech signals from you that the crowds are ok and not threatening. If you stress or get aggravated, then it will make him worse. If you have a group, you could always triangulate him, and keep him protected by family members so at least he is bumping into familiar people (though with siblings, that could be worse...) At the very least, you could give in to the smart phone and have him so distracted that you just need a firm hold on his arm to steer him through traffic he won't even see. Hey, no judgment - with sensory kids, you see to their needs and happiness on vacation, not a bunch of "shoulds" in parenting. I assume you have the noise muffling headphones for him. They are great for rides themselves, and may help in crowds.
A walking resort, or uber/lyft or Minnie Vans to and from MK or other parks could be nice as well, though I am not sure of wait times for rides around Christmas. It is often cheaper than renting a car, and you don't have to deal with parking and get door to door service. I don't think they are that bad, but post fireworks is anyone's game. Speaking of fireworks, my favorite for Fourth of July MK Fireworks is to sit on the grass at the Grand Floridian by the lagoon for an epic show, and you don't have the same crowds as in the park. You could do the same for Christmas and enjoy GF's gorgeous Christmas decorations. Another of my favorites was riding the ferry back from MK as the fireworks show exploded with the music piped in. Free fireworks cruise! Though a dessert party sounds pretty awesome and would be the best if you want to see the lights on the castle.
Yes, the crowds are bad, and sometimes the walkways are at a standstill. However, if that's when you can go, then just plan to manage the situation and enjoy it. Especially don't beat yourself up with "We should have gone at Thanksgiving again. Why did we do this?" Just have it be an exploration into the craziness of the Holiday Season around Disney, and chat it up with your family. Be ok with fewer rides and longer lines and be a rope drop maniac.
