Hey there, If you know he cheats on you all the time, honestly, how can you love him very much? You know he does it yet, you don't break it off with him. I know and him have a baby together however, cheating is unacceptable and something that nobody should have to deal with. You obviously have trust issues about your boyfriend as you did say you don't trust any female around him. Sorry to say this but without trust, there is no relationship. I do understand why you don't trust him though. I feel we should always have a little bit of fear in us that our partner could cheat on us but you know he cheats on you all the time.
Just about this girl he is talking too. I can't really say trust him when he says they are just friends because you shouldn't. He has been acting strange a lot lately and this is something you shouldn't just let go. After being together for 2 and a half years and having a child together, there should be trust in this relationship and there isn't any. You obviously have talked to him about this but try again. Follow your instincts here. Does he know that you don't trust him? Does he know you know he cheats? He may think that you don't know and can tell little white lies to get out of it when he can't. You really think they are more than friends so you should do something about this.
Communicate with him. Explain how you feel about this relationship, all the problems in this relationship and get him to understand you aren't giving him any more chances. I do really think you shouldn't after this. I will say it again, you know he cheats on you all the time which is why I do think that you should break up with him. However, I can tell you do really care about him. So, talk to him once more, lay down the law and if he picks that path again, end it! Try your hardest to get that trust back in your relationship. He doesn't deserve someone like you if he is going to continue too do that. If in the end, your relationship ends, he will always be in your baby's life so don't think he won't. He has responsibilities for your child.
Good luck!