He should be the one working at earning your trust.
So you spoke to him like shit? He treated you like total shit when he was fucking someone else!!
For me I would never trust him again.
How do you know he hasn't done this before? Just because you caught him this time doesn't mean it's the only time. Was his behaviour different after he had sex with someone else? Was he nervous/jumpy or on edge? Did he appear guilty in anyway? If he remained normal, calm and casual between the act and you discovering it, that suggests he felt no remorse and it probably wasnt the first time.