I’ve never met his kids, friends, dad etc. I eventually met his mum and her partner about 3 months ago and that was the first she’d heard of me. When he mentioned his 2 best friends were maybe coming over to see meet up with him next week for a ‘blokes’ night, he suggested I didn’t come over that night. He says they know about me, but I have no idea if that’s true or not
OK, this is your problem, not Facebook. If I were you OP I'd start a new thread about how, after 18 months, you're not really part of his life. At that stage with my partner I'd been away with him and his main group of friends, spent a lot of time with his daughter and his family knew about me and had for some time (for reasons of distance I hadn't met them).
And I think there is a generational problem with the responses. There is a difference between saying 'OMG, are we Facebook official' and just quietly updating your FB status to 'In a relationship'. I've never done the former, I did the latter when someone sent me a friend request and I suspected they were interested in me and fishing for information. It was a really easy, trouble-free, non-embarrassing way for me to say I wasn't interested.
The OP's boyfriend's FB does reflect something about his life - in this case he seems to be keeping the OP out of it. I know you hate confrontation OP but I would have a serious talk with him about where you are at. I have a feeling that whilst you are serious about him, he is using you as an 'ok for now' option. You really do deserve better than that.