I get it. When he doesn't know people, he relies on you to draw him in, remember he's there, smooth the social path and generally have his back.
When you're out somewhere that is a place he feels totally comfortable, he doesn't have a part of his mind doing the same for you - he just thinks about his own stuff and maximising his own social benefit.
I think you need to walk him slowly through what happens when he's at an event with your friends. "You went to the bar, so then you came back... why didn't you stop and strike up a conversation with people on the way back? Oh you didn't know them? So you felt more comfortable talking to people you knew well? Remember then I started talking to Flip and Flop, my friends, and I brought you into the conversation? You might not even have noticed it but I was making sure you always had a comfortable social chat, as well as making sure that I did."
If he acknowledges this you can say please could he try to do this more for you while you're out, as it is a kindness to you that matters a lot to you.
If he says he doesnt notice it and doesn't care and you're free to leave him on his own too, blah blah, you say yes but this is something that matters a lot to me and would be a kindness that means a great deal to me - it might not be important to you but to me it signals your love and care for me. Can you do it?
He would probably then bluster and say it's unreasonable of you to want it and it will spoil his pleasure... then you calmly say yes and this is really important to me because I love you and it's a way I'm asking you to show you love me. If you don't want to that's fine but it will mean that we don't go out together so often, which is a sad thing for our marriage. I am willing to try being more of an effortful social butterfly and giving you space at your events, so long as you make a similar effort for me.
Then it's in his court.