P.,
I am sorry to say that nine times out of ten, when a woman has the feeling that her man is cheating on her...he is. As for his side of the story, I remember a not-so-great time in my life when I dated more than one guy and would play them. I would literally have two of them standing face to face one of them thinking the other one was my gay friend, and the other thinking one was just a friend from work. I didn't do it to be mean, I just liked both of them in different ways and I didn't want to lose a good thing on either side...I also didn't want to get caught. I also dated a guy long-distance all through college and after, that was with another girl the whole time...he ended up marrying her so I was the fling. Is this other girl just the fling and is he living up to his other responsibilities as a father and provider?
That said, there are two things for you to consider. First is how long have you been together? If it just over three years, could this be something that had been going on before you that just continued, or was it something that happened WHILE you were together? The other thing is how old is your son? Did this thing get re-kindled or started while you were pregnant or after childbirth when you weren't feeling very romantically inclined? I do not believe that it is ok, but I do know that men have certain needs that will drive them to do stupid things to have those needs met (why is there prostitution?) and they justify it to themselves that way.
That said, IF you guys are back on track in the romance area, and IF he is treating you and your son well...it may have passed. Honestly, the only thing that bother's me is that he won't come clean. Trust is based on truth and he owes you that, no matter how painful it may be. If you really want to know...really want to know, here are some ideas: but before you try this, MAKE SURE YOU KNOW WHAT YOU WILL DO ONCE YOU KNOW (will you stay with him and work it out, will you leave him) if you are mentally prepared for the worst, you won't do anything reactive that you will regret later.
I was a cop and did many interrogatons. If asked directly, people would lie. They get defensive and they don't want you to be mad so they lie and lie. If you want to try to build your trust back up, tell him so. Tell him that you still have questions and that you just don't buy the line about the crazy girl. People will also take an excuse if you give it to them. Try to figure out a motivation then ask an either/or question NOT A YES OR NO ("I know I wasn't there for you physically, was it just a short-term physical thing? OR Was it just a one time thing with her?) The last one won't sound so bad to him if you put it that way as opposed to "are you cheating on me?" You can say "I know you never would have done anything intentionally to hurt me" which is a good line to use to take the responsibility off of him as does "so I know you wouldn't have usually done anything like that, did you tell her you were with me and she still seduced you? Is that what you meant by she was crazy?"
If you do get him to admit that he was with her, keep the gentle questions coming. If he says it was a one time thing, say "oh, come on...it had to have happened more than once for her to say that you were "with" her." If you are gentle but persistent, you will eventually pick through to the truth. He just has to feel that you understand why he did it (even if you don't and think he is a sorry you-know-what.)
I understand the difficulties you will face if you break it off, especially with trying to raise a child alone. You can stay in the relationship for your son's sake for now, but if you can't get past this girl thing, I believe you will just become bitter and angry in the long run and I believe your self-esteem will slowly be crushed more than it is already. That may work for a while, but eventually your son needs role models on both sides that will enable him to grow into a strong, self confident trustworthy man. Good luck and I will keep you in my prayers. Let us know what happens, we are here for you!