Boyfriend's friends stopped inviting him out

So my boyfriend and I have been together for nearly a year. I’m quite introverted and he’s extroverted and has a lot of friends and tries to include me a lot but I just prefer being at home alone rather than go out and drink and be around people in general. I try to make an effort for him but usually leave parties early whereas he’s the last one standing and I always encourage him to stay without me etc. Recently though his friends stopped inviting him out to places. We would always see his whole friend group hanging out whereas my bf wouldn’t even get an invite and he’s obviously very upset about that. We are still invited out once in a blue moon but it’s more for big parties rather than wee small things that everyone else does. For example, we’d get invited to big parties but not to grab lunch the next day where his entire friends group would be there. He’s very upset about it and I can’t stop thinking that I’m to blame for him not being invited. He says it’s not true but I can’t stop thinking it and it’s making me so upset to see him so upset and it’s making me feel extremely guilty and I don’t know what to do right now.
Sorry about the extremely long post I just feel so guilty about it and I don’t know how to stop the thoughts or how to help him apart. I love the guy to bits and would never want to hurt him so leaving him is the last thing on my mind but sometimes I feel like if it wasn’t for me he’d still be hanging out with him friends instead of me etc.