currently struggling with the same thing. one thing that has brought some improvement is explaining how his body works. we talk about the food going in, his tummy working on it, and it goes down, down, down and comes out his bottom. understanding it (to a degree) helped him to relax a little bit about it.
he likes to be alone, so my husband suggested putting his training potty in the corner (where he likes to go hide) or behind the door. that way, he can have the privacy and still poop in the potty. actually haven't tried that yet, but plan to.
good luck. i never had a college roommate who still wore diapers, so it will work out eventually!