You can either

a) tell him and accept whatever happens next

b) break it off with him

c) stay with him but don’t tell

I think it’s a little different since he specifically asked you. It probably means he has suspected foul play in the past and doesn’t fully trust you. This isn’t a good basis for marriage at all. If he hadn’t asked, and you had fully repented/moved on from some horrible mistake you made years and years ago I think option C would be plausible. No need to hurt him if you fully intend to love and commit to him. But since he asked and is obviously skeptical, I think option C is limited here.

Examine why you cheated in the first place? It doesn’t sound like you are in love with this man based on post history. I think option b is your best bet given the limited knowledge I have of the situation. If you stay with him though I think you guys need to talk about it.