I don't think of his as a tsundere, he just needs to rest and recover + has a calm personally. And as of now he's not really in love (hm, starting maybe). I'm pretty sure he'll be warmer when he'll be in a relationship
*him
and I also think Yechan's good for him / is a positive presence / has a good influence, that I definitely agree
Oh, actually "tsundere" was author's actual description for him iirc.
Yechan's stubbornness is exactly what Sunyool's tsundere needs.