If the kitty sneezes a lot, it does not necessarily mean he is sick. Sometimes sneezing is a perfectly normal function of the respiratory system. Of course, we cannot always assume this is the case.
Might be he is allergic to some fabric or cleaning product. A lot of cats are also allergic to Febreeze and other scents. Have you noticed if he sneezes a lot when you use a fly spray, use certain household cleaning products or when you wear a certain type of cologne/perfume that is a little bit too strong? Do you smoke? Smoking can also cause cats to sneeze. It could also be that this time of the year is causing him sneeze.
What you could so is start looking at the possible causes; plants, scents etc. Any allergens you can think of that’s being spread in your house, and get rid of it. If you have something in the house you think is causing these allergies but you can’t get rid of it, then I suggest you contact a vet to help you provide something for your kitty so he feels better.