Well you already did one thing wrong and the only immediate way to repent would be to do the humane thing and tell the wife. She deserves to know. Nobody deserves to stay and waste one more minute of their life with a cheater.
Firstly you should stop seeing said guy. He's married. You're a homewrecker. Would you like someone else stealing away your partner someday? I'm sure you won't. So don't do onto others as you wouldn't like being done to you.
You need to stop seeing the guy immediately! If you only want to tell the wife so you can have him to yourself, then get your head examined. If that's not the case, tell the wife her husband is a pedophile-adulterer. then pick a single guy next time around. Karma is a bitch.