How do I stand up for my step son without overstepping as a step parent?

My husband recently got split custody my step son. He's in the 8th grade will be in the 9th grade in August. We get him Monday, Tuesday. His mom gets him Wednesday Thursday, Friday and we switch off weekends. His grandfather lives with him and his mom and he started asking me if I would pick him from school and if I he could stay over until his mom got home from work. I said that was fine but the first day his mom got mad at me and said her dad can get him and that we don't get him on the days that are not ours. So I had to follow the custody agreement. She has made it clear she wants me to have nothing to do with parenting. He always gets upset when he has to go back to his mom's, but I noticed when her dad was out of town and he was going back he was fine. He asked me if he could talk to me and said he doesn't like being alone with his grandfather. When I asked why he put his face in a pillow. I asked was his grandfather mean to him and he shrugged. I asked did his grandfather hurt him and he nodded and I asked how does he hurt you and he wouldn't say. I spoke to my husband about this and he spoke to his son's mom and she said that he's probably just upset because her dad is old school. But I feel it's more than that. But I don't know advocate for him without overstepping.