I think that chimera Roxas should have an autoimmune condition
the themes are already basically there with the way his body is at odds with itself, so think should just bring it all the way home and make it an active autoimmune condition
PLUS! I think that Axel wouldn't really know about that being a possibility even, so he and Roxas just operate on this assumption that Roxas "gets sick all the time" and Axel thinks his boy is a trooper who gets over a cold in no time!
it takes Saïx pointing out the pattern of Roxas overworking himself and turning out "sick" that Axel starts to realize huh, yeah, it's almost always stress or over-exertion that seems to precede Roxas being sick, huh?
plus Axel has never caught one of Roxas's "rapid onset headcolds" in all their time together, which is starting to sound like something other than Roxas having a weaker immune system
this revelation would 100% incur Roxas bragging that "my immune system is actually SUPER STRONG dad! it's always working! it never rests!!!"