Is it possible that you are feeling insecure because of your current condition? or are you truly seeing it for what it is?
You have every right to feel secure in a relationship and to expect monagamy from your partner. Talk to him about it, but don't just ignore your instincts and bottle up these bad feelings. The real question really is, though, if he is what will you do?
If you are going to stay anyway, because if he gets caught and promises never to do it again he will find a way, then save yourself and your baby the stress and accept it.
But if you feel you deserve better then find out, even hire someone, or get a friend, or do it yourself and find out and nip it in the bud.
Is that wrong or sneaky? Not any worse than what he is already doing to you and his child. If you take his word snd still feel this way it will leak out and gradually create havoc within your family,
I apologize if this is so blunt, but women need to set their boundaries and stop acting like victims and choose to stay and deal or get out and stop creating more drama than is necessary especially for the kids.
Either you stay if he is and accept it, or leave him. The solution is simple, but the pain with either choice is what's difficult. Remember, the child is the one who will ultimately be affected. You cannot control your husbands behavior, but you can control your choices and your child's environment.
If you do give him another chance, that's fine, too, but if he does it again you took that chance and need to make your decision accordingly. It will not be your fault if he does, but you did set yourself up.
Either he will change or not. Are you going to sit around and wait to find out?
Just a quick tip: Men are simple. It is their nature, most of them anyway, and for them they like the immediate pleasure and they decide that, if they get caught, all they have to do is get you to give in. So they suck up all they can, and once things are comfortable again the cycle begins again.
But if you stay and make things uncomfortable then they will use that as an excuse to do it again and blame it on you pushing them away with your lack of trust. Don't get caught in the cycle. Either take another chance with him, or make your decision.