D.B.
Hi J.,
I know nothing about helping an autistic child learn anything but I can tell you how I'm dealing with my 3-yr-old. He, too, has no problem peeing in (or on!! lol) the toilet but asks for a diaper to poop in. I have no problem doing this as I feel he needs to advance at his own pace and when he feels comfortable enough with what must seem to him to be a HUGE hole (toilet) to sit on, he'll do it.
This is not an uncommon request from what I gather from friends and acquaintances; it's just how the diaper-request is handled by the parent that seems to differ.
In case you haven't done this, although I'm sure you have, : ) ask for guidance from his autism school. I'm sure they can give you helpful advice.
Good luck and God is truly blessing you!!
D.