You should definitely tell him. He deserves to know
Just send him an anonymous paternity test. It will sort itself out.
Dump the cheater and tell him.
You of course should tell, there’s a good chance she’s still cheating/will cheat again. Also if she’s got a kid there’s also a chance it’s not the guy who she’s with’s kid.
And fwiw idk why you’d stick with someone who cheats on you. You say she got away with it, it sounds like your bf got away with it too and you’re really the only one paying for what they did.
Edit: accidentally posted before I was done.
If you do, consider the consequences. 1st, he might not even believe you. 2nd, he might forgive her and he’ll possibly be put in the same mindset you’re in.
She might go running to your bf about you causing problems for her and your bf may side with her. However you spin it, it may not turn out well. And in the end, it still won’t give you peace.. Please take some serious consideration before making a decision. I’m so sorry for your heartbreak and pain. Neither of them are worth it.