It sounds like he’s being considerate here, by talking to you about it, rather than just ploughing ahead with his decision. It’s been a long time already for his GF and him not to see each other so I can understand why it’s come to a head now. I’m living apart from my DP and would move him in here in a heartbeat if he was living alone.
6 months is too quick to move someone in under normal circumstances, but this is not normal and we all have to do whatever we can to make it work. They’re clearly serious about each other to consider this. Being locked down isn’t easy for anyone, whether being forced to spend all day together or not seeing each other at all, so it may not be without problems, but what is at the moment?
His other options would be not to see her and allow her to become depressed and isolated, or to break the rules and meet up anyway while still living apart.
Would you be ok with the latter? Because that’s what many in his situation are doing. It’s just unfeasible to expect relationships to wither and die due to rules which don’t allow for any contact between people who are low risk.
I know it’s annoying - the idea that my DP can spend time with his ex and not me is so unfair, and I know she’ll be meeting up with friends and blatantly ignoring the rules, whereas I’m not, so am much lower risk than her, not going out to work, DCs staying home, only the odd shopping trip to expose me.
I think like all things shared-care, you have to be open to a sensible grown up conversation about it, listen to what he’s got to say, and realise that you don’t have a right to blanket ban anything he does on his time, just as you wouldn’t want him to have that power over you.
You have no automatic right to meet the GF before (or even after) she meets your DS. As hard as that is, so your only concern here is how it will affect your DS to have someone new around while he’s there.
Have the chat with him, express your feelings about it and see where you end up.
Personally I can’t help thinking your DS could benefit from another adult around, who will presumably make your ex happier and more fun to be around than if he’s pining away for her and worrying about her. Sad to say, dads often put on a better show of being a parent when they’ve got someone to impress too!