What a great update. I'm glad for you.
And, if you think you've gotten a good peg on the "my family isn't represented and I feel sort of alone" idea, you could find ways to help him with that.
Like, arrange for flowers or a big bouquet of balloons to be delivered to the party, right in the middle of it. Even if nephew doesn't really focus on it, all the other grownups in the room (mostly brother, but others too) will be reminded that yes, he DOES have a family.
I understand the "I'm outnumbered," feeling, and if the people doing the outnumbering are sort of enemies, it's even worse.
Or, you can suggest to him that if he and the boy's mom are split, it might be time to have separate celebrations. And he doesn't need to worry about the other side's being bigger; he can find ways that his celebration is pretty special.