Snakes are very difficult to evaluate health-wise, & I do not take any of my snakes to a routine vet check, nor do I suggest that you do, UNLESS you see symptoms
of a problem. We have no way to tell you for sure that what you're hearing is or is not the beginning of an RI...it's entirely possible that there is old shed or debris
in his nostril(s), but pay close attention since you also said his appetite isn't what it should be. It's a good time to double check your husbandry (right temperatures
and all that?) & if the problems (clicking sounds & loss of appetite) continue or worsen, you'd be an UNDER-protective "snarent" if you weren't concerned & ready to
take him to the vet as needed.