If you decide not to get your son right away, go to a woman's shelter first. Do not call the police, he has just as much right to your son as you do and with no custody order, it's his word against yours about what has been going on. The fact that he kicked you out of the home works in your favor in finding shelter, they will help you work out the rest.
IF you decide to go back, do with a definite plan to get out with your child, like, tonight. Stay within eye shot of his parents at all times so you have witnesses to what happens while you are there. Bring a friend with you if you have to, it's time to call someone to help. I know many are telling you not to go back, but he has your son and you know it's not safe for him there. Unless you truly believe that your husband will hurt you and/or your son if you go back, go get your boy and get to a women's shelter (or at least a good friend's house where you can stay for a day or two with your son) in your area immediately.
One more thing, you can't tell the school that he can't pick up your child unless there is a legal finding for that. You need a lawyer to put something like that in place. This is why finding legal help immediately is important.