My boyfriend has a son he doesn't see

I am 28 years old and have been with my partner for almost 3 years ago. He has 2 daughters from a previous marriage, whom I have known about since the beginning. Although this is difficult for me, we see them every other weekend and I am adjusting well to my role in the relationship. Last summer, my partner told me that he had another child, a son, who was about 2 years old at the time (would now be about 3 / 4). The son was a result of a 1 night stand and my partner has nothing to do with the child any more. When the mother fell pregnant, he told her that he didn't want the baby, but the mother chose to keep it having previously suffered a miscarriage. My partner met the son a few times and made a few attempts to be apart of his life before realizing that he couldn't love the boy like he does his two daughters. He thought that if his ex wife knew, it would affect his chances of seeing his children (she is very tempermental) so decided to keep it a secret from everyone, and his only involvement was to pay child support. After telling me this, I convinced him to tell his ex wife and that it needed to be in the open. I told him I couldn't be with someone that would deny the existence of a child and wasn't willing to be a part of his life. But then as time went on, I started to think how much harder everything would be bringing this boy into our lives and started to wonder if, as someone with no children, I could actually take on that much baggage. Afterall, the daughters were hard enough. I then started justifying it in my mind... Surely it was better for this boy to not see his dad, than a half a**ed attempt to be involved. Wouldn't it be worse on the child to see the relationship he has with his daughters and wonder why it wasn't the same. It made sense to me, afterall, had my partner of been the female in the relationship then he would have made his decision himself. I know it takes two to tango, but she had given him misleading information to think they were protected (this all hasn't come from him, I spoke to the mother myself when I first found out). We ended up carrying on as though nothing had happened. Seeing his daughters every other weekend, paying child support for the other child and that was that. Now it's a year on since I found out and I need to talk to someone about the whole thing. I know what the obvious opinion should be - he needs to man up, become a father, and I should leave him if I am not able to support it. That's not entirely useful, I would like to know ways to cope with this. Should my partner see this boy, what level of commitment needs to be made. Just any advice and help and discussion would be really appreciated.