DGS unhappy about convos not involving him

He is 9, he has presumably had lots of attention for 9 years and now has a baby (if I've read this correctly),

He is feeling pushed out of being "the little one" in the family and so now wants to feel like a grown-up.
That's what I think.
He wants to join in the conversation with your daughter and you.

I am meeting my daughter occasionally now when her children are not there so we can have a different kind of conversation.

Could be way off.

I'm sad that I'm not being given a chance to speak to him.

How do you not get a chance to speak to him? Can you phone him?

When you see DD, does DGS get your undivided attention for part of the visit?
If he does, then explain to him that you want to talk about different things with DD and you need to have some time with each of them.

When my young DGC came round with their parents either I or my DH was talking to them or playing games with them 90% of the time and the other one of us was doing the grown up chatting. Young children get bored with conversation and want to play or interact. I used to love playing games, Lego, playdoh, cars, cards whatever with my DGC.

Does convo = conversation?

HappyBut - poor choice of Username - sorry, just ignore me.

Convos?