I wouldn't go with those weird "special needs trikes". Go to the recumbent subforum to see the kinds of trikes most people ride these days.
A tadpole trike would probably have the coolness to appeal to him. Both of those are pretty expensive options unless you can find a cheap used one on Craigslist or eBay.
Only the OP would know, but its concieveable that the boy could even ride a bike. Perhaps start off with a used step-through 3 speed, remove the pedals and start him off using it as a run-bike, till he gets balancing it down.
It's possible that this would be a life-skill for him. When I lived near Palm Springs, I used to see one young man with Downs who would ride the bus, and when he got to his stop, get off the bus, dismount his bike from the bus and ride the rest of the way to his home. It also allowed him to go to the local grocery store.