Poopy on the potty

I have a 5 year old boy who is Autistic.  He has recently had bowl troubles.  He also recently started  going pee by himself in the toilet. My question is, he refuses to poop on the toilet.  He will always poop in his underwear and cries every time I put him on to go.  I usually put him on after he poops in his underwear because I know he still has to go, but I could have him on for an hour or more and he doesn't poop in the toilet.  I am frustrated does anyone have any suggestions?

[margypops]

He could be constipated and it hurts ask him if he has tummy ache, if he doesnt answer point to his Tummy and rub it to see if it hurts,If hes peeing standing up I have known kids finding it difficult to get it in their heads the Peeing is standing up and the Pooing is sitting down, let him do both Sitting down if you can, make sure his diet is good in case of constipation.