Can I tell you something that’s not gonna help you? Nothing you do is gonna stop his philandering ways. In fact the more you try to get into it, the more you’ll mess up your mental health over it.
And what can your mother do? If she’s not independent and making money on her own, divorce as a reality is so brutal. Courts take years, it seems redundant.
I would say work on yourself. Find such financial security that your mom and you can count on you and not him. Also make sure he doesn’t have any illegitimate children because those can succeed in property too.
Honestly you can’t do anything. Do not let his cheating rule your life and f you up beyond repair. I have been in your situation. For your mental health, let this not be a defining trait. Fathers like these are disgusting and I’m so sorry you went through all this.