Hi D.,
Not much I can add, generally they will do it when they are ready, not much point in trying to force things.
That said, I recall hearing of one little girl being trained to poop in the toilet by her parents telling her that her poops would like to go swimming! (the little girl loved swimming, herself) Now I can't say how that would go if a kid thinks the toilet is flushing away part of him/herself.....
Another idea is to have him observe a peer pooping in the toilet--neighbor child, cousin, family friend, whatever. He may think "Well, if s/he can do it so can I".
It's good he is able to communicate with you that he wants the diaper, rather than just messing up his underwear!!
fwiw, my oldest wasn't consistently doing both in the toilet until somewhere between 3.25 yo and 3.5 yo. And he was quite advanced in other respects (colors, vocab, right/left, etc--yes, I know I am biased!). But once he was trained there was little or no regression--not even at the birth of little bro when he was 3.5 and little bro being in hospital for 3.5 days soon after birth, and a move to a new state 5 weeks after that.
Hang in there. Maybe you should keep a journal of your efforts--in any case it will be fun to share with him if he has kids one day!
K. Z.