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
Felidae
Holistic Blend
Natural Balance
Newman's Own Organics - Cat/Kitten
Pinnacle
Solid Gold
Wellness
Innova
California Natural
TimberWolf Organics - Serengeti Herbal
Blue
BrightLife
Second Grade Cat Food:
Nature's Recipe - Adult
Sensible Choice
Lick Your Chops
Natures Variety
Third Grade Cat Food:
Annamaet
Artemis
Breeders Choice - Avo-Derm
Azmira
Bil - Jac
Diamond
Eagle Pack
Excel
Flint River Ranch
Hi-Tor
Natural Life - Adult, Kitten
Nutro Max
Nutro Natural Choice
PetGuard
Precise
Premium Edge
Prism
Pro Pac
Pro Plan
Professional
Purina ONE
Royal Canin
Triumph
VeRUS
Wysong - Uretic, Vitality
Authority
Maxximum Nutrition