Go to the emergency room, tell the doctor what happened.
It is important they get DNA proof and proof of penetration. If not your step dad can deny anything happend and he will try to turn your own mother against you.
It is the doctor's legal responsibility to report this to the police. This man is sick and needs to go to prison where he can't get to boys anymore.
If you are worried about your mom's reaction, I think she will be first shocked, angry, maybe blaming you. Don't put up with it, in time she will be glad you did it.
Afterwards, it is indespensible that you recieve treatment for PTSD, or your life will be ruined for many years to come. Please don't neglect this part.