I would say move your son to stay with a family member close by so you know he will be safe and for you to move out as well. Things like that start off small and just get worse, i grew up with my dad hitting my mom and i hated it, my mom left him and then years later had a step dad, he never hit me but just to spank me if i did something wrong , when i grew up he started to actually hit me and wanting to fight me over dumb stuff, i wouldnt fight back just push him away and let him tire him self out, well one day he was really mad and hit my mom then thats when i went after him, but anyways i would say alot of step dads are great but when little small things start to happen usually it means big trouble in the future, put a restraining order against him to keep you and your child safe.