jes: Don't switch foods too much.
There's a possibility he's allergic to something in the ingredients. Try a lamb and rice formula as apposted to meat by-products or chicken and corn recipies.
I also encougage trips to the vet 2x a year, to help head off any problems, instead of letting some health problem develope for a full year before you catch it.
Try not to let him eat too fast, feed him peice by peice by hand.
Elevate his bowl, put it on top of a phone book, something so that it makes his food more level with his head so he doesn't have to bend down.
All of the above might help. But also bring him to the vet.
Here are some great foods, some of which you can find in various formulas for our pets which are allergic to preservatives, beef, corn..etc.
Dry food only...
Frist Grade Cat Food:
Chicken Soup for the Cat Lovers Soul - Adult, Kitten
Evolve - Kitten, Maintenance
Newman's Own Organics - Cat/Kitten
TimberWolf Organics - Serengeti Herbal
Second Grade Cat Food:
Nature's Recipe - Adult
Lick Your Chops
Third Grade Cat Food:
Breeders Choice - Avo-Derm
Bil - Jac
Flint River Ranch
Natural Life - Adult, Kitten
Nutro Natural Choice
Wysong - Uretic, Vitality