Hey there
! I find Disney transport extremely easy to use in my wheelchair - almost all of the drivers and CMs I had have been delightful; the procedure is well-practiced, and normally over in a flash; and everything is planned out to make it as simple as possible for everyone involved.
If you do use Disney transport, I'd suggest you use the standard wheelchair boarding procedure, rather than carrying his chair on. Even though it's a pediatric chair, it will still be reasonably heavy and bulky, and I'm sure the last thing you need is to put your back out getting it on and off the bus
. Also, if you turn sharply carrying the chair, or slip and fall on the steps, you might end up hitting somebody with it, if they were a bit too close to you. If you use the lift / ramp you will have as much time and space as you need, and it will be much safer for you and the other passengers.
Is your son's chair designed to be strapped down in a vehicle? If so, it might be easier on you all to have him stay in the chair for the journey. Staying in his chair, he'll probably be more comfortable, have any specialist seating he needs and will be less likely to hurt himself if the bus breaks suddenly. I don't know whether he has any issues bracing himself on a seat without restraints, but as Goofy's sis said, they have seat belts available for wheelchair users. There is an optional shoulder belt, but I'm guessing he's probably too small for that to be comfortable. You can still use the lap belt, without the shoulder strap. Strapping the chair down, also means that it won't go off and 'explore' other parts of the bus if the driver has to break quickly (I've tried holding a folded wheelchair on the bus, and I've nearly had mine escape a number of times
. One of the many reasons why I stay in my chair now).
As Goofy's Sis pointed out, park closing is about the busiest time for the buses. If you want to stay for this, you might want to consider moving fairly slowly towards the buses, taking in all the sights on the way. We find that if we wander through the stores on the way back, the bus is normally much quieter than if we rush straight there (also, the shops are usually nice and quiet at this time). The other time that the buses get really busy, is when one of the water parks has to close, due to storms. At this point, everyone empties out of the water, and head for the buses. OK, people will take different amounts of time getting changed, but that's still a whole park full of people to pack into those buses
. If this happens, you might just want to head to one of the food places and grab a snack or lunch. By the time you've finished, either the storm will have moved on or the crowds will have died down.
All the other transports in Disney, such as the Monorail and the boats, are also accessible. The only vehicles I've found that were not accessible, are the smallest boats that putt around on the MK lake (the ones that look like they've come out of 'Steamboat Willie'). If you want to catch one of these, you can fold and carry the chair on; it's usually one or two small steps down (depending on the water level), but the gate in the railing isn't wide enough for an unfolded chair to go through, as far as I remember. There are usually the bigger ferry boats running the same routes, which are fully accessible, but you'd need to check that, as they change their scheduling every now and then. For all accessible boats and the Monorail, it is usually a small step up (usually around 6" - 8", though this can vary with the water level - it can even be a step
down if the water is low enough) which they can put a ramp to if you wish.
However you choose to play it, have a great trip!
but we can be pretty sure they would not want a wheelchair to be loading onto a full bus with more risk of hitting any feet or leg. 
to disABILITIES!
