Nephew hitting classmates

I’ve been very involved in my nephews life until recently. He just turned 6 years old last week.

He lives with his mom (my brothers exwife who is remarried) since my brother lost custody about 2 years ago. I was living out of state but still tried to come see him as much as possible and I’d always stay with them.

Over this last summer he was at home with his stepdad with not a lot of activity other than going to my dads house once a week and swim lessons once a week.

The week he started school he hit a classmate. The following week when he went back, he hit two other kids. And about two weeks after that he hit another kid.

His parents are keeping him basically on lock down and not allowing him to go anywhere. I’ve only gotten to see him once in about 6mo and they won’t let me come over or pick him up at school. His mom works full time and so he’s home with his stepdad all the time.

I feel so bad for him I don’t know what to do. His stepdad yells at him and picks on him a lot. He’s completely negative towards him. I feel like keeping him there is the worst thing they could do.

I would love any suggestions on anything I could do to help him. I’ve offered to help multiple times and his mom never responds back to me. I feel stuck and i want to help him so bad.