hmm.
if its a constipation issue (which isnt neccesarily not going, but painful passing) simply using milled flax seed - you can put it on just about anything. it doesnt affect the taste, but it might add a little 'crunch'.... yogurt, cottage cheese, on cereal, in sandwiches.. whatever. its basically fiber, and some people will say "well, its not good to hae to get used to using something like that in order to poop" but thats a lie... we NEED fiber in order to have healthy stool!
fiber absorbs water and makes stool softer and easier to go.
basically, im thinking maybe he had one bad experience and hes afraid of it happening again? maybe even had diarhea?
anyway, it sounds like you are doing the right thing; not pushing him, not making it a big deal. take it easy on things that constipate, and follow your instincts and his lead...