He’s Reluctant To See His Son

haven’t confessed this to anyone else, but my youngest son has let himself and his family down badly. He has a son from a previous relationship and his dad and I are really close to the boy, helping his mum out with childcare when we can. My son puts in minimum time and effort with his child and would rather be with his new partner. If I try to talk to him he accuses of me of taking his ex’s side. How do I make him see I’m on his side, but he should make more effort?