I think you do know what to do, but it’s not what you want to do. 🙁
Your boyfriend and the father of your child together has been cheating on you, probably the whole time he’s been with you, and he’s still doing it. He doesn’t want to be monogamous to you, and he’s not going to be. Your friend that he’s flirting with (at least) and possibly sleeping with (at most), is not your friend. Friends do not treat friends the way she’s treating you. 😕
So, my advice to you is to end your relationship with this guy. He’s going to bring you nothing but heartache, and as your child gets older, your boyfriend is going to be a very bad role model for your son together.
Go to court and get a child support order, so that he will be ordered to support his son, and start your life as a single mother. Now. While it may seem heartbreakingly hard, it will be much easier than what you will endure if you stay with this guy who has no respect for you.
I’m sorry that this isn’t what you want to hear, but your girlfriend is the least of your worries. Your boyfriend is the problem.
Good luck. 🙂