who makes an ultralight with 30" door?

The Keystone Outback 240URS might work for you. Should be towable by a half-ton, it has a smaller ramp door on the side next to the main door so that your son doesn't feel like an outcast being sent to the "back of the bus", and plenty of room for him to maneuver into bed. The only real problem looks to be that the dinette and couch area are kind of cramped, so that may limit maneuverability. But the front bunk area appears big enough to set up a separate accessible table.

http://www.keystonerv.com/outback/#/floorplans