I don't know what to do about mine either, he makes me feel like I'm intruding when I'm at their house. He never has much to say, although I try to have a conversation with him, it soon peters out.
I never take sides, never interfere in their life, I'm good with the two GC, I just give up trying to engage with him now and let him do his thing.
My daughter admits he's hard work and says he's shy but he has a high grade job so has to deal with people on a daily basis so maybe he just doesn't like me 
