Calling mom's SO "grandpa"?

Question

So my mom has been with her SO for several years and they are not married, but they consider themselves to be. He is 14 years younger than her (whatever floats your boat) and I've never had an issue with that. He is only 9 years older than me and I've never, nor will I ever consider him a "father figure" or refer to him as my stepdad. He's her fiancé, and that's it. We have a decent relationship, but I do have my own doubts and uncertainties about him. 
While I was pregnant, she sneakily expressed a want for him to be my sons "grandpa". And has mentioned it here and there since he has been born (He is 4mos now). While I could see why, I'm not comfortable with this for several reasons.

1. He is not my "dad" or "stepdad", therefore he isn't my child's grandparent. 
2. He has not expressed any interest in my son whatsoever. My mom has tried to get him to hold him several times and he always ignores her and brushes it off. 4 months and he hasn't held him ONCE. 
3. If you aren't going to love and be supportive and BE THERE for my child, then you don't deserve the title of a grandparent. 
4. He isn't going to call someone grandpa just to avoid it being uncomfortable for my mom. It's uncomfortable for my husband and I if he did, so not everybody gets to be happy.

My question is: How do I have this conversation with my Mother without hurting her feelings? I think she will take it personally and I don't want to start a fight with her. She has to see what I see, right? Maybe she's in denial because it's what she wants so bad, even though he isn't showing much interest...

Thanks for reading to the end. I know it was long 😒
If anyone has been in a simalar situation, please share what you did or how you handled it.

Answer

All of the points you wrote are valid points. #3 especially.  You need to tell her what you told us. Good luck with that conversation.

Answer

I'd say exactly what you wrote. He's your son & your mom will have to realize she doesn't have any say in what he calls her SO. My grandmother has a boyfriend & they've been together as long as I remember. My grandfather died before I ever met him. Everyone calls him "papaw", but I refuse to. He's never had any interest in me or even once acted like my grandpa. That name is reserved for the grandpa I never got to meet. The rest of my family still gets mad at me about that except for my parents who feel the same way, but I don't care. That's their problem.

Answer

My grandfather has a fiance, me kids will call her grandma sue even though they aren't married.