i know you feel very strongly about him, and i in no way discount that. whatever age you are, your feelings are valid.
that said, you need to cut it off with him. never, ever, ever stay with someone who has assaulted you. it's not too late for him to change, but going back to him after an event like this will reinforce the idea that it is okay to hit you, because you will come back, and you deserve better than that. it doesnt matter if you went on national tv and insulted his mother, how he reacted reflects on him, not you.
i'm also worried about him being intoxicated. if he has to use substances to get through the day, he's likely an addict. its not as though addicts cannot be good people, or get better, but an addict is only as mature as they were when they started using. if he started drinking heavily at 13, then he's going to be as mature as a a 13 year old until he quits and works on himself.
remember that, no matter how much you love someone, it is never, ever, ever, EVER your responsibility to make them better. not if you love them, not if theyre your mother, not if its the president if the united states, no one. you can be supportive, and encourage him, but 1. theres no guarantee theyll do want to get better, and 2. i would feel very uncomfortable asking you as a minor to even know how to navigate that relationship. I'm not saying you're not mature, seeking counsel on this issue shows me you are, but he is in need of professional help that not even his grandma can give him.
in my thought, the best thing to do is break up. meet him in a public place or over the phone and communicate how you feel, but enforce your boundaries. if you say you dont want to talk to him, then dont respond when he tries to talk to you. tell him you cant help him, and then dont.
this is hard. i hope you both come out the other side aright.