I'm very worried even though he's happy he eat chicken breast and his cat biscuits no problem.he's also sneezing a lot and has been for months
Sometimes cats just stop liking a food. It may be the cat being a cat, or it may be that the food in question changed the recipe (an all too common issue), or even that they just made a batch with slightly different ratios that the cat dislikes. Your cat may also have become allergic to something in the food, which may make your cat stop liking the food.
With that said, given the age of your cat, and the fact that it is multiple foods and brands, the most likely cause of your cat's issue is medical, and you should take your cat to the vet (probably with a stool sample if you want to be proactive) ASAP.
Also, cats need actual cat food, not just cat treats and chicken. Without cat food, they won't be getting numerous essential nutrients and will sicken and eventually even die of malnutrition.