Welcome to the forum!
With the popularity of many open floor plans you should be able to find one. The biggest problem you may have is modifying the entry door and steps to help him enter and exit, but this is possible.
A Fifth wheel tends to provide the most room but you might also have to spring for a bigger tow vehicle. If you want a larger bumper tow, then you start running into lightweight (and more fragile) materials since most tow vehicles have lower tow ratings for a travel trailer than a fifth wheel. I've towed both and much prefer the fifth wheel. It takes a little getting used to the large object behind you and you must worry about height much more but you'll get used to it quickly, learn to use your mirrors better and will be more rested since the fifth wheel tows easier because of how it's carried in the truck.
Since you have a permanent arrangement, many long term RVers build and install permanent ramps to the door way. I'm not sure how or if they modified the door, but I have heard it's possible to have it modified to accomodate a wheel chair.
It's great to hear your son enjoys camping. That's more than half the battle and whatever expenses are done to accommodate him will be worth it.
