S.,
You and your son are not alone out there!! Most kids go through some sort of regressions or fears around the potty. I don't know if any of these ideas would work, but here goes!
1. Let him go back to pull-ups for when he poops.
If it's become a problem at school (ie. the teachers have told you he's asking for a pull-ups, soiling himself and/or they want the problem fixed or he has to leave), you'll need to work something out with them during this (completely normal) regression or look for another school.
Otherwise, let him hold it at school and use pull-ups at home. The more relaxed you can be about this, the faster he will probably grow out of it.
2. Some children become afraid of going poop on the potty because it hurt once when they were constipated. My dd hated going poop in the potty for the first month or so of potty training and she would hold it for as long as she could.
I took some advice from mamasource and I made her poop as loose as possible by giving her lots of fiber, fresh fruit and water. It made it impossible for her to hold her poop for long, so she would eventually use the potty.
The difference is that my dd hated going in a diaper or pull-up.
In your son's case, you might just not offer a pull-up when he asks for one. Feed him lots of fiber and water, and then just tell him you don't have any more pull-ups. Don't be upset or angry if he poops on himself - just clean him up (he's too young to clean himself up) and say something like, "maybe next time you can use the potty!" Try to act really calm about the whole thing. This wouldn't work at school, but if he's holding at school and he only goes there 2 days a week, you might be able to get this to work at home.
3. Use the reward chart idea that the pp noted - some children respond to this, others don't. If he's afraid of the potty or of going poop, this won't work.
4. Ask him to help a stuffed animal use the potty. Tell him that the animal is afraid to poop in the potty and ask him what you can do to help the animal. This works great with some children.
Well, that's all I can think of! These setbacks are soooo frustrating, but this, too, will pass and he WILL be using the potty again. Best of luck!