I am sorry he did this and he puts her child first, its not right and I know how you feel. Is this other child his as well or hers from another relationship? My ex is doing this now and it is the most baffling, cruel thing a father can do to his own kid. I am lucky my boy is too young to understand and only met him when he was a baby, thank god.
My advice? stop messing about on his facebook page and stirring things up. It only hurts you in the long run and from my own experience just makes matters worse. He is a grown man and if he is behaving this way, let him, he will miss out, not you.