It's extremely rare for it to be okay to out someone. Even though this guy is being shitty, the potential danger you're causing him is not something he deserves. A lot of what happened probably stems from his fear of coming out, due to what has likely been a very homophobic upbringing. Don't out him.

It's best to just move on.

Your boyfriend cheated on you, right? Just put all of these people behind you by breaking up with him.