You don't give very much information. How are your parents abusing their power? The court decided in the best interest of your son and included your parents partial custody. Don't begrudge them this, because they also love and care for him. If you and the child's father are able to fully support this child, and have no involvement with drugs or a record, then by all means go back to court for full custody. Until then, be grateful your parents are helping. They don't have to, you know.
I would think that sharing custody as In 50/50 would entitle them to as much power as a mother or father would normally have. In y state that would consist of not crossing state borders without permission and things like that.