Nutrisca salmon and chickpea (no potato, no grains) agrees with my dog that has a sensitive stomach,
or consider homemade: http://www.homeovet.net/dynamic/php/downloads/dog-c8470f2c75dbe4b683205c3919ee2310/dog_diet_complete.pdf
You may find some helpful tips in General Guidelines
Other options, consider a homeopathic approach: http://www.vitalanimal.com check the blogs for related topics
If you want to stay with the traditional vets, ask for a referral to a specialist.
Get the condition properly diagnosed, his symptoms may not have anything to do with his diet.
