My son refused to potty train until he was about 5. At the age of 6 he started showing some anger issues and was smearing fecal matter all over the walls of the bathroom. (We also are a very loving family). Alarmed, we took him to a child psychologist where he was diagnosed with Aspergers. Coincidentally we found out my husband had Aspergers also, as there is usually a direct genetic link, Father to Son. My husband appartently wet the bed until he was about 11.
It's hard not to blame yourself, but don't. There is something going on with him, and whatever it is, it's probably not the end of the world. It could be something as simple as fear of what goes on in the bathrooms at school (and those can be very predatory places) or some other condition.
If you can get him to talk about it, and it is as simple as the "scary" bathroom thing, then you could work out some solution, like having him get a hall pass after he gets to class so that he can be alone in the bathroom.
The other suggestion would be to have him professionally evaluated. After we found out it was such a relief. My son had been exhibiting some "different" behaviors from his pals, so this let us work with that.
Now that he's 16, there are a myriad of challenges but at least we all know what's going on. And now we know why Dad can't look anyone in the eye and is basically socially challenged. Sometimes when they are both driving me nuts, they get in my face and remind me that they are both retarded and that I should be a little more patient. It's a personal family joke, but since they are both "high functioning" Aspergers, I'm actually the retarded one in the house LOL!! (I don't mean to offend anyone with the "retarded" comment, it's just our family's quirky sense of humor).
This is a challenge, and you are doing the right thing by not stressing him out. This must be horrible for him, poor guy. Let us know what you are going to do/did about this, I think you could help other parents dealing with this.