My son is 4, almost 5. I went through EXACTLY what you are going through now. I started potty training my son by the time he was 2. He seemed like he had all the signs and was ready to go. At first, everything seemed to be going well. Then we moved...not once, but twice. I noticed that during those times or times of stress, he backed up and we would have to start all over again with training him. I would get so frustrated.
Sometimes, the stress of something major can cause them to have setbacks. Sometimes those set backs go all the way back to the beginning! When my son and I moved here to Sequim, WA...that was the worst setback. I practically started from the beginning. But now that we're settled in, (we've been here a little over a year) he's done great! I have him fully potty trained! Even at night! :D It could've been the move and school that had your son regressing. Something that might help, is to watch his cues and have set "potty" times. (I.E. When he gets up, right before/15-30 min after a meal, before a trip...etc.) That always seemed to help for us. And if you are at home? NO POTTY PANTS! I hated those things. They didn't do anything, but hinder his progress in the day time and help out a bit at night. Hope this all helps! Good luck! ;) ~B.