There a two possible reasons why he is a kofer.
He has no desire to believe in God, in which case you (no offense, but you know its probably true) can’t do anything about it.
Or, he rationalized that God doesn’t exist. (Now of course you say, “No! It’s not rational! Believing in God makes more sense!” Yes. Shkoyach. I agree. But people still do this.) But this also stems from a certain element of arrogance and close mindedness as well. If its the latter, call up someone who’s an expert in kiruv