what do i do?

so i recently found out my boyfriend of two years (age 23) is hanging out, texting day & night, went to the beach and she “showed up” and god knows what else with an 18 year old who just graduated high school. he tells me they’re just friends and so did she but they’re messages say other wise. so we broke things off but he’s still calling/texting me and we still hook up. what do i do????? every time i try to stop he blows my phone up and flips everything on me. he thinks i’m talking to another guy, he freaks out. i’m at a lost...help.