He's a fuckboy and an asshole. He doesn't give a shit about feelings and love, he dates to and get his wet. You're his newest victim. You've been together for five months, it's insinuated you're a nerd or an outcast in some form. Either way, you haven't fucked yet. So he sank his into someone else and you just walked up and overheard him admitting it.

Break up? Slap him? Be spineless and don't reprimand him.