[SRW]

I think this was perfectly handled, as others have said. You saw that he wanted to make the change, you allowed him the opportunity, then when you saw he didn't know how to admit that he forgot to re-enter the starter, you prompted him with what he needed. By doing it the way you did, you averted a clusterf*k later on with a potential unreported sub, and allowed the coach and the player to keep playing.