Assuming you're American, you have the benefit of at-will employment. That means unless there's an implied contract or you're in one of the five states where it'd violate some public policy, or he's somehow about to get tenure, you can fire him for whatever reason. if you're worried about it looking like discrimination, the easiest answer is financial motivations.
"I could split up your wage into raises for the rest of my employees in return for having them do everything you do and it'd get me the same results of right now, so I'm letting you go to save money. Adios." That is a completely legal and justified excuse for firing someone. You could also just lay them off and then never hire them back - though I'd recommend laying off a couple of people and not just him to cover your ass.