He may have feline asthma, lungworms, or tumors and/or growths in the airways.
Try sitting with him the bathroom when the shower is running to see if the steam can relieve some of the wheezing.
The only way to know for sure what is wrong with him is to take him to the vet.
But, if he doesn't seem in pain and he hasn't had any asthma attacks then I wouldn't worry too much for now.
If you notice a loss of appetite, foaming at the mouth, or losing weight I would seek veterinary attention ASAP.