Just a few thoughts on him wanting to use a contact centre - it does make it more formal and you're right, a benefit is that this will be harder for his gf to meddle with. We don't know if that's how he sees it, let's hope so, but it will be interesting if she persists with the constant messaging during the sessions. It will be good to have someone there from ss to witness his reactions to this if it happens.
I do have some concerns that this decision also relates to his last visit to you though, because (and this may be a bit hard to hear, so sorry for that) whilst entirely reasonable for you to want to explore things with him (it seemed like no-one else was) he had said previously he didn't want to discuss his relationship with you when he visited. The conversation you had with him about his friends, and about the girl partner situation did encroach on this territory. I expect the gf made a big deal out of this with him, but it may also have made him feel like his boundaries were being overstepped. With someone in an abusive situation, these feelings can be exacerbated across all relationships and it's common to then become a bit hypervigilant about other people's motivation. So in some ways it's positive that he's recognised this, and is able to put a boundary down, even if a bit clunky, but in others it is emphasing mis-placed blame that you are being portrayed as the abuser here.
Of course I may be entirely wrong, but I'm just saying this might be something to bare in mind as you consider how to approach contact. Hopefully, now there is a professional on board, exploring and working with his feelings about the whole situation, it will enable you to feel less pressured to find out exactly what's going on for him (because someone else is) and the risk of you overstepping in this way is reduced. You can concentrate entirely on just enjoying some time with him, reminding him you are safe, for the first few sessions at least, and just see how it goes.
If I am right, and this is behind the decision, it may pay off , further down the line, to acknowledge and address this with him. I do wonder if family therapy might also be useful to you both at some stage, maybe chat to your own counsellor about this.
Hope you're feeling okay today and not too wiped out after the meeting. These things can be quite exhausting so again, take good care and be gentle with yourself.