The ex wasn't abusive. He harassed him a few times after they broke up, but it seemed like that was all. At first I thought he was just taking advantage of him for his money and didn't actually care about him, but then he talked about how when they were together he wanted Serizawa to say sweet things and fight for their relationship, like he's doing now with Kaito.
I think it got tagged as abuse by mistake.
Found this manga at "donesric abuse" tag. What so abusive about this manga? Its fluffiness?