It’s a difficult one. The phone surfing must have some sort of impact on your son as he’s probably noticed he’s not getting the positive attention (from your partner) that he gets from yourself each time he does something new, says something new etc. As
I’m sure you know, children love positive reinforcement and love praise, like clapping, cheering, laughing and smiling when they’ve done something. He knows he gets that from you. I’ve been guilty of it myself of being on my phone when my son is playing, until I come to the realisation that my poor boy is playing on his own, it’s awful really as you don’t get these years back. I’m sure you’ve spoken to your partner about it countless times, telling her and getting her to understand that these are the most important years for your child and your child craves the attention from his mum even if she doesn’t think he shows it at the moment. She will then get the attention she wants from him. We made a pact of never being on our phones whilst he’s in the room and focus our attention on him. She may see it as some sort of break and alone time whilst you’re watching your son, but I’d say that’s what’s impacting the way he is with her compared to you. And if she wants anything to change then she must be prepared to make changes. Not sure if any of that waffle helps.
Take care.