Wow, you must have had a painless divorce, because I think most people after such a traumatic experience, would avoid drama-filled relationships at all costs. I have been going through a messy one myself and no way in hell would I get involved with someone who has as much baggage as your boyfriend, someone who moves so fast as to act like you two are walking down the aisle soon, despite being together only 4 months, and who jumps from relationship to relationship. Think about it. This woman was with him in August, and you and him have been together for 4 months, so how long was he single for? Less than 4 months. That right there should have been a red flag, assuming the other 2 kids and 2 marriages were not. You obviously are aware that this guy has character flaws, or you would not have said "(yes, I know...)" when discussing his past. You know the answer as to what you should do.
For me, peace of mind is paramount, as is stability. There is no way I could have peace of mind with a man who barely has time for me and my son, because he has to take care of his 3 other families, and who knows if you'll be number 4 soon? I don't see how this man can maintain a healthy relationship, yet still be present for all, and hold a decent job so he can meet his responsibilities. What would happen if he lost his job? Would you be willing to pay his child support for him? This guy sounds incredibly irresponsible and immature, not to mention, impulsive. He is trouble and you know it, but you keep making excuses for him. You guys need a responsible man to step up and be a man, not this man-child, who sounds like a player, sleeping with everyone in sight, without using protection and then....oops! And no, I don't believe for one moment that he just found out she was pregnant. He was probably waiting until you fell for him and knew you'd have a hard time kicking him to the curb. Who knows if he was two-timing, considering his lack of responsibility in making children, I wouldn't put it past him. No woman is going to keep quiet about needing financial and emotional support until she's at her last month of pregnancy.
Why do you think you and your son deserve so little? I would not have introduced this man to my son in the first place after dating for just 4 months, but you already did so. Yes, you know the answer, and it is, to let him go and be a father to his 3 kids, without having to juggle you and your son into that mess as well. Elayne J. said it very eloquently, as did B. in her post. He needs to step up to the plate and grow up. A vasectomy sure wouldn't hurt. Be more selective about who you date in the future. You may also want to think about using protection, at least until you find someone you'd want to settle down with and have more kids, assuming that is something you're seeking. The last thing you want is to be one more baby mama in a bunch, and fight the rest of them for child support for your child, when the money can only be stretched so much. Good luck. Get out now before more time passes and you and your son get more attached to this guy. Nothing good can come out of sticking around and waiting to see how he juggles you and everyone else.