DS (4.8, ASD) refuses to poo in the toilet. He was toilet trained earlier this year (no problems, very happy to take himself to the toilet to wee, happy to flush etc), but has always waited until he has a nappy on in the evening before pooing. Sometimes, if we are later than usual getting him ready for bed, he will go in his pants. He is fully aware of what he is doing and knows we want him to use the toilet (as we're changing him, he'll make a comment like "if I want a new train, I have to do a poo in the toilet"). He also hates being dirty and asks us to change him straight away.
We've tried rewards, but he will only consider it worth his while if the reward is really significant like a new Hornby train, which isn't financially sustainable! He loses motivation if the reward isn't instant, so sticker charts (with the promise of a present after several stickers) don't work that well. I'm at a loss as to what to try next