CY-Ren can turn people into familiars too? [Damn, that means there's actually more people affected than he thought. Maybe they'll turn back when the Harbingers they're tied to do.]

I, uh... lost a fight with Steven. [Technically Yuya wasn't fighting back, but 'got beat up by' just sounds terrible.] But he didn't do anything to me even though he could have, even just to get rid of a Star Guardian... I don't think he was trying to take you just to have more people on CY-Ren's side.

[He's tired of feeling hurt by people, but he hasn't forgotten his friends, has he?] Even if it didn't work the first time, that doesn't mean we shouldn't try again. This time, we know what doesn't reach his heart, right? If we can understand and speak to his feelings, too, I think he'll listen.

Then you can remind him that he doesn't have to feel so hopeless about his future.