He is upset that you 'officially' have a boyfriend, with an idiot. Damian doesn't like that another person is the one who is close to you. He wants you all to himself, although he won't admit he's jealous. How to say it? He's going to seem like an obsessed guy, and of course he doesn't want that.

“You could break up with him.” Damian hissed with contempt in his voice. It doesn't sound like a suggestion, it sounds more like a demand. “Right now.”

It's because you should be his, only his. Damián can't stand seeing you with someone else who doesn't deserve you. His blood boils just thinking about it. But it hurts to know that he doesn't really have the power to stop that, it's your relationship, your decisions. Oh, how he wish to replace that idiot.