Mum and partner don’t get on

From day one both my mum and partner did not get along, and now me and my partner have a child it’s even more complicated.

Everytime I visit my mum with our son he refuses to come and then moans that he will miss a day seeing his son?

Or if my cousins wish to see our son and my mum they would plan to meet me at my mums and again he would have a tantrum about how he won’t get to see if son all day?

We live together? He sees him everyday after work and weekends? I don’t understand the concept of him moaning he doesn’t get to see him when we all live in the same place?

Does anyone else have a partner and mum/parents that just don’t see eye to eye? And how do u deal with situations like this?