It seems that no one thought of this: At school he is expected to be a "Big Boy". he Has to use the potty there because that's what Big Boys at school do. But, hey, he's only 2 1/2 years old. He's Not a big boy. Maybe he still wants the comfort of being the "baby" at home. None of us wants to grow up, right? Any one of us adults would like to be pampered and toput off responsibility. Especially after a long, hard day at work. We don't mind doing chores on our days off (like he Used to use the potty at home), but after work we want to relax.
Cut him a little slack. He's doing a Great job where it counts--out in the big world. He feels safe at home and that's where he can let go a bit (pardon the pun). Heck, you're lucky! My first boy didn't potty train until Much later. :)