Your boyfriend is absolutely right, what you are going through is abuse. We are happy your boyfriend stood up for you, but are even happier that things didn't turn physical between him and your step dad- that really could have made things worse.
We know that it can be scary to call child protective services or somebody else, but it really is a good step to take. This man has been harming you physically and verbally- neither of which should ever be done under any circumstances. If you don't feel comfortable calling CPS, you could always speak with the school counselor, a teacher, a relative, or any other adults you trust. Let them know about what is going on. Any way you choose to open up to somebody is better than leaving things unsaid.
Abuse usually just continues and often times gets worse; we don't want to see either of these things happen to you! You boyfriend saw what has been happening, he is a witness. Please don't let the fear of your parents denying everything stop you from opening up. Have you spoken with your biological dad about what your step dad has been doing? We bet he wouldn't be too happy to hear about it. Or, you could talk to somebody from your biological dad's side of the family (if your father has passed away).
Stay strong. We know how difficult it can be to experience abuse, but you really are showing a whole bunch of strength and courage by reaching out for some support and guidance. These are amazing traits to have. Please continue using these traits to get some help from other adults in your area who can help put an end to this abusive behavior at your house.
Keep in touch; we are worried about ya. Please let us know how things are going and what steps you would feel comfortable taking.