I think Nanjo is a softie deep down but the problem is he just can’t trust Mokyeon. Time and time again, he kept changing their terms to outright breaking their rules, so either Mokyeon needs to prove he can be trusted. The other way I can think of is Nanjo approaching him again for sex because quite frankly Mokyeon taught him a whole new sexual world and might not be satisfied with anyone else physically anymore. The second option might be interesting because Mokyeon who used to be a fuckboy would be going through his personal hell to be with Nanjo only physically and not emotionally after discovering love for the first time.
Slight spoilers maybe but my interpretation of Nanjo’s reaction is more like he’s asking if Sunhwa’s okay out of general concern and not because of any romantic feelings. Him and Mokyeon did, technically, go behind Sunhwa’s back together. Overall though, the vibes from him is that he’s so over the messy ass situation.

Read the chapter in Lezhin because I couldn’t wait and I completely find Nanjo’s thought process realistic because at this point all he wants is to forget that everything ever happened. He’s trying to blot out Sunhwa and Mokyeon from his memories and it really sucks for Mokyeon, but genuinely Mokyeon hasn’t given Nanjo a reason to want him in his life. I can’t see a Nanjo chasing arc happening, at most it will be Mokyeon trying desperately to prove himself and Nanjo feeling pity (and also the undeniable lust that Mokyeon trained him to feel).