I'll start by saying, I'm not an expert. But, at home I'd recommend letting him go bottomless 24/7. If he makes a mess, he needs to clean it up (with your supervision and possibly help, of course). Put potties all over the house. They sell some cheap (like $3-4) at Ikea.
At school, I've heard they potty train by taking them to the potty on a very regular basis. If they go, great, if not, that's o.k. too. I guess the thinking behind that is that it becomes routine. As I'm sure you know by now, with special needs kids, routine is critical.
Don't expect him to learn in a week. Since he's starting quite a bit later than the ideal time, he's already trained his body to let it out in his diaper. He'll have to break that habit and it will take a while. I think having him bottomless will help him learn when his body needs to eliminate. Yes, it will be messy at first, but you need to stick to it (remember, routine), don't turn back, don't use pullups, just move forward, and the accidents will become less and less. Good luck!