forget him! don't reply, and avoid. if he cheated with you, he'll cheat on you.
this once happened to me in that i got with someone i knew not too well under the impression he was single, and when he then revealed he had a girlfriend i felt guilty for ages! felt so bad about the girl, i know i would feel awful if i had been cheated on, so for the girl's sake if noone else's, stay away from him!