M.C.
He is kinda young... It is great that he poops in the potty, but he might just not be as motivated to pee because it happens more often, and it is a more frequent interruption to his day. Also, since he is in a pull-up, there are absolutely no consequences for peeing in his pull-up, because, after all it is just an expensive diaper. You could go either way...
1. pull back to diapers (more economical, since this could go on for years) and let him tell you when he has to poop in the potty - I would still encourage that behavior.
OR... 2. lose the pull-ups all together and work really, really hard to motivate him to pee in the potty. This will take lots of effort for both of you, so brace yourself.
Now, in either case, let him know what you are doing. Go shopping and buy him the most awesome underwear that he can find. If choosing option #1, just put them in his drawer and tell him that when he decides to keep his Barney Undies clean and dry, he can choose to wear them. Other than that, we go with the diapers. One day, he will be ready and he will put those undies on and it will take him 5 minutes to be potty trained. But, that day might come when he is 4 1/2 years old.
Good luck!