collect evidence, call the cops. with a child involved i'm not sure the legality, but if it's joint custody you can bring it up with the court if you feel the child is in danger and you could appeal for full custody. there's also changing your number, blocking him online, and moving, but again, with a son involved it might not be so easy [especially if he has some form of visitation rights]
magickally protection spells are a good idea, however these work better for negative energy, not negative people. banishings would work better. they probably won't result in him being flung from your home, but he would feel uneasy around you and want to leave you alone.