Advice on His First Grandchild?

My partner and I have been together for nearly five years. He does not want to get married. Ever. His daughter just announced she’s pregnant. This will be his first grandchild. He keeps insisting I’m going to be a grandma. No, I’ll be Grandma’s girlfriend. I texted his daughter asking how her appointment today went. She left me on read as she has for every message sent since November. An hour later, she texted him a photo of the ultrasound, labeling it “Your grandchild.” He excitedly showed me. And understandably so. I don’t share the same enthusiasm. I love babies but this isn’t my grandchild. It’s not even a relative. So I’m feeling a bit neutral. And I feel a bit guilty about that.

Because the grand baby is coming, he’s also decided that he can’t join me on my next career move which would bring me closer to my kids.

Babies are reasons to celebrate. And I know how happy he is to become a grandpa.

Any thoughts on giving him the space to celebrate his family’s new addition while respecting my own feelings through this, too?