@SightWarden It's even more nuanced than that: He feels his interacting with Hina is emotionally damaging to her. Much like a recovered alcoholic often swears off touching any booze lest they relapse, Ren feels if he distances himself from Hina, she won't be tempted into that self-sacrificing mindset she gets into around Ren. I don't know how much he knows about the bullying, but he likely doesn't know what else to do. And as far as he knows, Hina doesn't really get anything out of the swap, as she's essentially "bodysitting" for him. (Although I do find it nice there is the small joy she's found in playing sports as Ren and hanging out after school, even if she probably feels really awkward about being a boy)
I'm just afraid this is going to end with Hina Changing with Ren and jumping off a roof to "force" Ren to be "happy" by living as Hina and marrying Makoto.
Hopefully the nurse girl can help the two of them.