M.J.
A meeting should give everyone a picture of what's going on.
I think the regression in potty training, or the fact that he is not using the potty at school, is a sign. One thing about those older classes at daycare is that they have more kids in those rooms and that might be an issue with your child. He might feel like it is just too much - too many kids, too much noise, too much waiting his turn, etc.
Have you talked to him about it and asked how he feels? That might shed some light on what you should do.
M.