We have a name we call my Mam, it’s done with affection and my mam laughs signs bday cards etc with the nick name, if dd called her it, she’d laugh. It’s a silly name like the OP.
However, my Dad isn’t like that and would find it rude and would leave and would tell everyone about what a shit Daughter I am and how awful my child is. I repeated a joke I’d heard by adults in his presence at a similar age and got similar treatment to you son, the joke is one I’d heard from his own mouth ffs and at the time I had zero understanding of why it wasn’t appropriate. He also expected hugs and loving affection, didn’t like that I teach my dd her body is hers an nobody is owed physical affection from her. I could go on and on with a million examples.
My Dh respected how I felt about my (now dead) Dad and I’ve cut contact with his wife and his stepson several years ago, he wouldn’t ever push me to see them and trusts me to know what’s for the best, he doesn’t want our dd being made to feel the same way my Dad made me feel as a child.
I do get what you try with them, but I’d be listening to the man who was raised by these people, and they’ve shown you they haven’t changed, I’d be making sure it’s the one and only time they make my son feel like shit. I’d not want it to be one of several memories like that.
A small genuinely warm loving family where you can be yourself is way way better than maintaining the relationship between a large family where you have to feel anxious of people falling out and getting offended etc.
If dd said something She didn’t realise would upset my Mum, She’d say sorry, my mum would say it’s ok and it would be back to normal.
If dd did it with my Dad, she’d say sorry, Dad would draw it out and store it in his memory bank to bring up when telling stories if his badly done to he is.(he went in and on about my mam not pairing his socks and what a shit wife she was until the day he died, they’d been divorced over 30 years ffs)