Patience - I have to say I do agree with most of the other posters who have said that your ex was hardly "tricked"... if he didn't want kids or, more importantly, to pick up an STD, he should have worn a condom. They teach that to 10 year olds these days! There is just no excuse really. I think the whole "tricked into fatherhood" thing is a load of baloney - as women we really shouldn't let that sort of nonsense survive! Women don't manipulate men into becmoing fathers (just like evil, nasty women don't tempt perfect husbands and fathers into affairs...). some men are just too lazy, stupid and irresponsible to put on condoms.
BUT, I totally get that this situation has thrown up a whole load of conflicting feelings for you.... I am glad you are seeing someone to work through them. Probably your feelings are a bit "dog in the manger".....? You don't want him as such (being married didn't work), but you still want to be the special somone (the special friend) in his life..? I am not judging - its really hard to let go of people, even when you know you should! Also, if your ex has a good relationship with your child (his ex-stepchild, I guess), maybe you are worried about that being replaced by his relationahip with his own child...
But your ex seems to be clinging on to this slghtly unhealthy relationship too... you say that he told the new girlfriend (or whatever she is) that she will have to accept you being part of her child's life. WHAT? You are just friends with your ex. You will not really be part of his child's life with another woman, will you?! This isn't your fault of course (he said it!), its just clear that he is a bit weird about your relationship too....
Agree with other posters that, if you really are just friends with this man, you should take a step back, tell him (he is your friend, after all, and we can be honest with friends!) that he has been a right twerp and he needs to man up, and let him get on with getting to know his child when it is born. Maybe you will occasionally see the child if he happens to be looking after it when he sees you (as friends!), but more likely, you won't and will have very little, if any, reason to be involved.
Hope some of that helps!