I'm in the same boat I'm 16 about to be 17 and my boyfreind is 20... my mom approves of it but my step dad don't so can he do anything?

What state are you in?

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

Are you in TX also?

But yes, there are things your stepdad can do until you are 18.

Your step dad has no legal standing in this, since he has no legal relationship to you. He could have course report that he suspects sexual contact between you.

But that will depend on where you live, In states where a 16 year old has age of consent, there is nothing he can do.

If your mom or bio dad did not agree even if above the age of consent, there are things they can still do using other laws that restrict other adults having control over you.