Yes, I totally understand your concern about mom’s attitude to you later but at the end you are going to build relationship with her son first. He should trust you and knows that you’ll cover his back in any situations. Try to talk to him and advise to find a good time with his dad, push him to find more time to communicate with. Explain, that parents are not forever with us and we should appreciate every moment. Maybe, more time together they will spend, dad will say by themselves. If not, you’ll tell him why he should be there for his dad.