K., he is probably still very resistant to go in the toilet because he thinks it will hurt. I had a similar problem with my son at about your son't age. Here's what I did. I found this solution on the internet. I think if you google 'Dr. Kroll' you can find it...
First of all, you need to make sure that he doesn't get constipated again and that his BM's are very easy to push out.
Start him on mineral oil (1 tablespoon in the am, and 1 tablespoon in the pm, and expect a little orange colored leakage in his undies) and tell him it's his tummy medicine so he poops don't hurt him. I mixed it in a little OJ in a medicine cup and my son took it with no problem. Also you can start adding fiber like benefiber to his drinks. With these two things it will make it very difficult for him to hold his bm's in. You must do this every day so that he gets used to having a bm that 'feels good'.
When you can see that he needs to go (pacing around, holding his bottom etc) put him in a diaper and encourage him to go there. Then you and he can dump it in the toilet together. Put him right back in his underpants after he goes (I'm assuming he pees in the potty with no problem)
Then,the next time encourage him to go in the diaper in the bathroom, standing up, or whatever. Once he does this a few times, tell him you are going to make a magic diaper that lets his bm pass right into the potty. Take a diaper and cut a good size hole in the bottom. When he has to go, put it on him and get him to sit on the toilet. After he goes, show him the magic! If he will do this a couple of times, you shoudl be able to convince him to sit on the toilet without the diaper. I got to this stage and bribed my son with a DVD, and that was the end of the problem.
I know this is lengthy, but it really works well. You don't want him withholding his bm's when he's 5 or 6; much easier to resolve the issue now.
Periodically I still give my son (he's 5 now) mineral oil, when I can see that he hasn't gone in a day or so...
Good luck!
(I just re-read, and I can't tell if your son is peeing in the potty or not. If not, then you should back off, like the other moms suggested. Stop pressuring him to use the potty, and keep him pull-ups. Leave it for a month and then try again with underpants. I would continue with the mineral oil and fiber though...)