He panics fr, mostly internally, but you bet your bottom dollar he is spending all his money if need be to fix them asap. If it's something minor he is taking it as an opportunity to be their savior, also Nik ALWAYS ends up getting sick. MC might insist that he not get sick too, but he'd literally rather die than not be able to kiss and be close to them, even if they had like a deadly virus, if you're going down, y'all are going down together.
When he's sick he's pretty self sufficient when MC not around, but when they are he is begging for cuddles and stuff to heal him, and he might like die inside if MC avoids being close to him as to not get sick, so he might pretend to be healthier than he is.