You didn’t like being bullied and mistreated by them, so I’m sure your son doesn’t like it either. You have every right to not let them see your son, and you don’t have to see them, either. My father was very abusive to my siblings and I, so he never been allowed to see my children, and I haven’t seen or talked to him for over 20 years. As an abuse survivor I feel like it’s my job to protect my children from it happening to them. Don’t let your son continue to be bullied.