The only thing to stop it now is his character. It is quite possible that his experience in the war and the responsibility and love that comes with a baby have helped your guy grow up. It’s at least worth giving it a chance.
Since you two are in your 30s, I wonder why it is that you haven’t made the commitment of a marriage. I suggest you talk to him about whether his attitude about fidelity would change if you were married.
It’s a good sign that he hasn’t contacted the other woman since he came home. Re-entry after a tour of duty can be challenging but it can also be an opportunity.
I wish you well.