I feel what your saying but if his sexual desires are that strong it won't work and he shouldn't even bother he will end up cheating and that's not fair to the kid or the mother.
That's y i can't stress enough to dudes have fun do you till you feel satisified enough to where it won't be a issue. Granted he got a kid now but he still can be a good father just don't be bringing them females around his kids and keep his issues to himself and not tell his baby momma at all. YOU NEVER STAY WITH SOMEBODY JUST CUZ U GOT A KID IT WONT WORK
Just to correlate what a few people said that I think are good ideas:
1) OG move like one poster said is to control your side piece. She needs to know she is just your side piece before you get involved, she needs to know that your BM and kid are your first priority
2) You could be open with your BM. Tell her you were feelin' her when you first met her, but she looks a lot different now. You don't want to cheat on her, so if she could get back on her fitness routine, you wouldn't feel temptation as much. You could maybe "accidentally" slip in some sidepiece talk to see if she's down for an open relationship that doesn't interfere with the household.
3) You just break up with her
I definitely think after a woman has a child, she is in her most fragile state physically, so that's when she's hoping the most that you as her man will still make her feel sexy and wanted, so this whole sidepiece thing is coming at a bad time, but hey man, gotta do whatchu gotta do sometimes