I'm with lisianthus, I have bad feelings about all of this.
So ExH can't contact, speak to, call or visit without the support of Saint Girlfriend the Great?
BUT, make sure your DS has funds to cover his own expenditure (her words) and I don't want my kids being bored by the long journey to come and get him.
If this donor of a dad can't actually communicate himself, live, in person, without SuperGirlfriend, then it seems like he's trying to ride his big boys bike before he's ready to take the stabilisers off.
I rather think that his sudden attack of conscience has got more to do with her telling him not to be such a dead beat dad, and shaming him into it, than him actually wanting to do it for himself/his own son. Or he's given her the impression (falsely) that he wants to be a proper dad and have more contact, because he thinks that's what she wants, or ought, to hear, or else she'd think he's an arse too and kick him out.
Either that or with her 4 kids, he's feeling a little outnumbered and wants to even up the teams a smidge...
I don't think she IS overly keen, perhaps she is trying to re-establish contact for some right reasons, but the comments that give her away the most are the ones about the money and the inconvenience to her kids.
I don't think she is doing this for genuine reason, and I don't think he's going along with it (for that all he IS doing) for the right reasons either. He's not battering down your door demanding to see your LO under any circumstances, he's allowing other people to do all the arranging, all the liasing, all he has to do is turn up. NOT acceptable.
I worry that this kind of insincere and ulterior motovated contact could be harmful for DS. I know the boy seems to be amazingly mature, and does seem to have a astonishingly real grip on things, but it still must HURT to have such an arse of a person needing to be handheld to be his father.
I'm afraid, FB must go, for a while. All contact must be between ExH and you, and you alone. you've said that you both can talk and get on... rather too well for your liking at times but this is something that MUST be sorted out, to your utter satisfaction before directly involving others.