He is crossing a line that he knows should not be crossed.
You are now at a point where you need to do something as soon as possible.
This is a dangerous situation to be in and your mother needs to take this seriously. If she does not, then you need to tell another adult quickly.
Your bad feelings are right. Get out. There is no other explanation for his behavior except that he thinks of you in a sexual way.
Im sorry to be so honest with you and I know its hard to hear but it is true.