Please cut this off now. With the exception of the child, I've experienced the same things. My ex best friend of the last 4 years did the same with me that entire time. I thought we would be together if he would just end his relationship, but guess what? We weren't! For a few months after ending his 6 year relationship he told me he didn't want another girlfriend, ever. Then a few months later he had a girl that made him feel better than anyone he'd ever met (yes he told me this even though he knew of my feelings because i was his "best friend"), but he still felt the need to be with me, too. 3 relationships ended without him wanting me for anything more than an emotional and sexual surrogate during and between the "loves of his life". I got away. It's not something anyone should have to deal with. It most likely will end the same for you. I think you should get away without telling him of your feelings. I know my pain got worse when I told my "friend" about my feelings. I know you probably won't listen to me, but you have much going for you to ruin it with this. I wish every single day that I could go back and change things. Cut them off early. It may seem hard, but it's easier than it will be later.
Also, please don't let him get you pregnant. Obviously, this isn't going to be what keeps him around, as he has a kid with the girl you want him to leave. It'll cause you so much pain, that you'll barely be able to stand, but can't get away from because you'll want your child to know his/her father!!!!