Ok, have read the whole thread now.
From my perspective, it sounds very much as though he started the relationship with her as a reaction to your telling him to leave. He wanted a shoulder to cry on, he didn't bring to it any sense of responsibility or giving, just taking from her when he needed someone. That is not a fair or good relationship.
That is what it sounds like.
He was also still very upset over you although doubtless there were things before you split that were not resolved, and not your fault, which is why you asked him to go. (I can't remember the ins and outs of it but just that I'm sure he wasn't the blameless party!)
The shit hit the fan when she conceived, perhaps he wasn't careful as he half (unconsciously) wanted a baby with her in order to hurt you/persuade you you should have kept him, ie in anger toward you - but I think basically he doesn't want to give the girlfriend anything, nothing at all, not a baby, and not his part in bringing up that child etc. He knows it means giving and he is not prepared to do that.
My guess is that he will not stay with her.
It seems very obvious from outside, (so may be wrong!) that he wnts to be back with you, or just alone, with possibly someone to use on the side, but no ties or children with her, and you want him back which he may or may not know. This is wortrh a try with some counselling but be patient as he may realise for himself that he does want to try again or may just be too proud to admit it.
FWIW, ex#1 didn't tell his wife I was pregnant - I didn't trick him, though he made out I did - and only after they had tried counselling etc etc and he had fought for his marriage, did he settle for me and the child...briefly, and never living with us, and then he turned out to have been sleeping at someone else's house and they later married. He has never seen Ds since. He told himself it was my fault and therefore not his responsibility, I think.
I have also found out that he has pretty much abandoned his older children as well, and just lives with the new wife and her three. I think he feels safer without the obligation, ie he feels he is doing them a favour so is thus not 'trapped'.
I digress...
Ex#2 was all for another baby once I had told him it was stupid to carry on seeing each other if he didn't want one, because I was desperate for one more.
When I told him I was Pg, his first words were 'fuck off', spoken in a shocked tone, and then silence for a few weeks aside from a few tears and sulks on my sofa and marching me down to the marie Stopes clinic to arrange an end to it.
It was cruel and awful.
I ditched him, relied on parents yet again...
He made various half hearted attempts to get back with me, but they were so pathetic and obviously under pressure from his mum, that I refused.
He called to arrange to see Ds2 last June. We made a date. I have not heard from him since, he didn't show up. He never wanted more kids, and hsas followed through about 'I won't be around to help with this kid' as he said at the start. What a crock.
HTH in some way!