Because your mom is being physically abusive, and you say the verbal abuse is really terrible, you should call the child abuse hotline and try to leave, or at least as soon as possible.
But, as a 17-year-old, you're young yet. Your mother may be harsh and possibly abusive, but online relationships CAN be risky. Your mom does have somewhat justified concerns.
The guy could be using you. How long have you known him and how far does he live from you?
There have been scary people using girls on the internet.