What I would do
This is a role playing game and turn it into a role playing experience. As a CE warlock, determine where he got his powers from. If he changed Alignment, have him re-examine if he wants those powers from that dark place. Most good people I think would say no. Then have him adventure to find a new source for his powers. If he used his powers while he was finding this new source, I might have him do damage to himself in addition to the foe. Or if the source that he got his powers from tries to stop him, his minions would be immune to the worlock. It is a game that is only limited by imagination, not rules.
This is a role playing game and turn it into a role playing experience. As a CE warlock, determine where he got his powers from. If he changed Alignment, have him re-examine if he wants those powers from that dark place. Most good people I think would say no. Then have him adventure to find a new source for his powers. If he used his powers while he was finding this new source, I might have him do damage to himself in addition to the foe. Or if the source that he got his powers from tries to stop him, his minions would be immune to the worlock. It is a game that is only limited by imagination, not rules.