my oldest son is also hfa(he is 8 now). we potty trained at 3.5, it took about 5 minutes, and he has never had an accident. i tried every few months from the time he was about 2.5 years old. the summer he was 3 was when the first Cars movie came out, and he was ALL about that movie! i took MAJOR advantage of that, and bought several packs of Cars fruit snacks, as well as socks, t-shirts, pajamas, toys, etc. One morning, I told him we were no longer pottying in the diaper anymore, poop and pee would be for the toilet ONLY, and every time you potty, you will get a prize from the box... then i showed him the box, it was a large moving box filled with Cars items - he was able to select anything the first day, following that - pack of fruit snack for pee, a larger toy for poop. by the grace of God, that was all the motivation he needed. he NEVER had an accident from that moment on, EVER! my non-autistic son was harder to train, we did a similar Wall-E theme prize box, but since the obsession wasn't as intense, he was a little more "whatever" about it - but he did train, just had some accidents along the way. if he's really into Thomas, and finances allow, make him up a Thomas prize box, you can prob find fruit snacks, candy, underwear, books, little trains - check out the dollar spot at Target, they have different character items from time to time.