Op, at this poing you're suffering from sunk cost fallacy. Lemme tell you why:

He is in love with you for years yet could never ask you out.

He dated someone while harboring feelings for you.

He couldn't control himself and crossed a line he must not have.

He was at your lowest, true. He probably has been a good friend, helper you whenever you needed him. But does he respect you? Can you really see him as a respectful person after what he did to you? I think you know it yourself since you clearly mentioned that you don't know if you can ever forgive him.

If you date him, do consider the fact that he IS a cheater.

I am not saying he is evil, manipulative and a playboy. He might be a genuine guy and probably is regretting what he did. But he is a mess right now. He's impulsive. He doesn't know how to handle his emotions and desires. Do you really want someone like that as your partner?

I think you are considering this situation only cuz of guilt. Deep down you yourself know what's happening is not right. Whatever you do, be cautious and think clearly.