Brown poop is good. It means he ate something of substance and there was enough of it for there to be waste.
White poop is usually just urates. If your snake is never defecating real poop (the brown stuff) it means he's not eating enough (or he's sick, but I think your vet ruled out parasites, etc).
You keep feeding him hoppers regularly, he'll be ready to move up to adult mice in a few weeks.