I need help with my step son!!!!!!

My step son is 5 and has severe behavioral issues at school and when he's with me. It's so bad that as a kindergartner he has been moved to an alternative school because the normal public school could not deal with him. He hits, throws stuff at people, pinches, screams bloody murder, and just a whole list of crazy things. But he's not at all like that with his dad or grandmother! I watch him every day after school while my husband works till 2 a.m. I need some advice on how to deal with these blow ups. Today he's doing it because I have him picking up toys in his room and he lost it and thankfully he's only 5 because he has resorted to basically physically assaulting me.