A little over a year ago, you posted about your son mooching off you, living in your condo on your dime, and dating a financially irresponsible woman with mental health issues. You, and everyone else, warned him that the relationship was toxic and you hoped he wouldn't stay with her, giving her money, and perhaps getting her pregnant. You worried that kicking him out of your condo would make him desperate.
Now, 14 months later, he barely getting by (you don't say where he lives - still in your condo?), and he's in another bad relationship with a woman a little older and with the baggage of 3 kids. I don't know whether you kicked him out of the condo and he hooked up with a woman who has a house, or whether he overreacted to everyone talking about his immature girlfriend and raced to one who seemed more stable, with a family. Maybe she is needy and strokes his ego because she can't make it without a man (you said she's recently divorced and now has brought a younger man into the lives of her 3 children - which isn't so smart), and if perhaps it feeds his ego to finally have someone who says he's capable and competent. Just raising points for you to contemplate.
I don't know everything that happened in between, but I'm going to guess that your son has shown signs of immaturity and poor judgment for many years, including during the time he lived under your roof as a minor. Maybe he didn't. But here's the bottom line: your son is immature, somehow got into his mid-20s without any sort of relationship (was he sheltered? just not interested? constrained/restricted?), and he cannot manage his finances. Maybe he's just enamored with sex and freedom. Maybe he is insecure and can't be alone. In any case, he is extremely limited in real-life skills related to work, social and sexual relationships. He doesn't have a long-range plan, he doesn't have any sense of budgeting, and I'm not sure he knows what's involved financially in supporting 3 children (not to mention the potential long-range repercussions if he has a child with this woman).
If you don't want to let him fall & fail, then the absolute only thing I can think of is a full-scale intervention with family and good friends with (and this is vital) a highly trained professional facilitator. But don't do it halfway or think for one second that you can do this yourself. Even without knowing what role you and his father have played up to this point, I can say for sure that this situation is well beyond having well-meaning amateurs manage it. I also think you should get some counseling to sort out your feelings and make some sort of plan about how you're going to handle him and what you're doing now that maybe should be changed.