Hi, Edwin,
I've got to think your son will be pretty miserable in school if they start him at grade 4 math when he's already finished grade 4 EPGY math. It's sort of giving them a curve ball. Will they just reevaluate and place him appropriately? Or allow him to continue with EPGY as the online course you mentioned, as he seems to love it so much? It would be nice if they continue to accommodate him - after all, a lot of the difference is pace, as opposed to knowledge. Hopefully, they'll let him continue at his apparent lightning speed. I wish my DS loved math the way yours seems to, lol!
Good luck!