Why are you asking us what you should do? Since you've come here to ask strangers, I'd say that you're looking for permission to stay with him even though your gut is telling you to dump him but you're afraid to because you've practically grown up with this man and you're afraid to be without him.
Its your decision what you do. I will say that if you want to be sure, then you will do your own sleuthing here and ask for proof that he went to a lawyer (there should be and invoice of some kind he can show you) there should be a paternity test done to prove the child is indeed his and this woman isn't just a scammer and anything else you can think of that would help you to prove his story as correct.
When it comes to infidelities, then I think that he should be considered guilty until proven innocent when you have the upfront information that you currently have. Your choice what you do with that information.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion