First I will just say that personally, I don't think it should be legal for him to buy an AR-15 as it's just a toy for guys who want to play soldier and/or crazy people who want to hurt lots of people.

Having said that, I think you also need to look closer at why you're scared of the AR-15 in particular and examine if it's really a rational fear. You already had guns in the home, so if your boyfriend was going to shoot you, he could use another of the guns instead. If you're truly concerned that he's going to use it to hurt someone else, that should be an issue you discuss with the police, not reddit. Frankly, if you have either of those concerns about your boyfriend, you should not be with him.

If the issue was that he didn't have ANY guns and bought one even though you had told him you didn't want guns in the home, I would be 100% for making him get rid of it, but I'm not sure that "I'm okay with guns in the home, but not this specific kind" is really a reasonable stand to take.