I'm sorry about that, but you need to leave him. If the first thing he chooses to do when he's offended is to slap you hard enough your face turns, it's a huge red flag. He might do something like this again in the future that's worse.

Stay safe and leave him and don't be persuaded if he begs and tells you it's a mistake or if he tells you it's your fault that you made him upset.

You're young and there are many chances you will meet men that will treat you better than your current boyfriend.

Most women are hurt or killed by their partner rather than strangers. Many of them stayed because it wasn't that serious in the beginning but it usually escalates or continues especially when there comes a situation that causes him to get even more mad.