He should be eating.
I would take him to the vet and have him checked out. It could be the respiratory problem is making him lose his appetite?
Try feeding him a bit of chicken and rice, and see if he will eat that. If he eats, then start mixing in some of the good kibble in. You could also try pouring some warm broth over his food...sometimes if it's softened and warm...they get hungry.
How much is he eating? Is he drinking? Is he peeing and pooping?
Gina