I think that the more this bothers you, the more hot water you're going to get into with your current wife. She will not understand that your friend violating the "man code" is the reason for your anger. It will seem to her like it is a jealous reaction. The more effort you put into "dealing" with this, the deeper the hole you're going to dig with the woman who you are now committed to.

My advice, forget your exwife and friend. According to his MO, he's going to have sex with her if he can and then he's out. If his friendship is important to you, then let it ride.

All of this will probably provide an excellent opportunity for you to discuss ethics, fidelity, and tolerance with your son regardless of what your ex does.