Oh, my ... I do remember how frustrating that is! Wow, though ... you should be happy that he is doing so well at 2.5 ... the average age for boys to be potty trained is 39 months!
Anyway, my son had that problem as well. From all I have read and all my pediatrician told us is that it is completely normal and will happen when they are ready. The poop thing usually does come later than pee. For my son he thought that sitting down to poop would hurt. So definitely make sure he has a step stool to rest his feel like another mom said. I have friends who swear by the following method. Have him sit on the potty to poop with his pull up on to get used to sitting to poop. I guess that happens for a day or two ... Then cut a hole in your pull ups so that when he sits on the potty the poop will go into the toilet instead of the pull up. Obviously once this works move towards taking pull up off all together. Sounds strange, I know and we did not end up doing this, but like I said, I have friends who swear by it and have actually read about it, too.
Another mom mentioned constipation. Do watch out for self-constipating. My son started to do that and the pediatrician recommended Miralax to get things going. We didn't have to do that either. I hate to say it but luckily he got a minor stomach bug and had no choice about letting it go in the toilet. From then on it hasn't been an issue.
Rest assured, it will happen and I know it isn't easy, but be patient. Good Luck and congratulations again on having an "early" potty trainer :)