Actually, this season only focuses on Kimoto's perspective and leaning to a lot of traumatic events in his childhood with Jin and other kids.
So, who knows what happened to Jin when Charin died since his perspective isn't really brought up.
This is like a 3 month time skip and grief is not always visible. Also he's not the main focus here. So he could be grieving but it's not important this season. That's all.
We did learn that Jin is really good at masking. When he was with Charin, he purposely took on a dumber persona. I think he's doing something similar here. He's putting on a carefree persona to mask his real thoughts. We're also seeing this from Kimoto's perspective and Kimoto sees Jin as a stupid idiot.
Bro why is he not sad about charin’s death at all. I hope he’s just pretending because there is no way he moved on so easily right? i want charin and jin to be end game at least for him not to move on THAT easily