this website help me to suggest best dog food for sensitive stomach.
i also recommended you to if your dog suffer in any problem so this is good solution and advice also
http://patmypet.net/top-10-best-dog-food-for-sensitive-stomach-2016/
Try Pro Plan Salmon for sensitive stomachs.
Ask your Vet about giving OTC Pepcid and the dose.
I agree with Jerry_NJ's suggestions about the probiotics and changing dog foods. Google dogtorj (not a misspelling) website and the GARD diet.
I'd suggest adding Probiotics which are supposed to help with digestive issues.
You may want to switch from chicken too... try salmon or even beef, and grain free. I understand he doesn't seem to have trouble with his dog food but who knows?
Probiotics are available as a supplement and in some dog treats. I don't see them at my local PetSmart, so I buy probo treats from Amazon. And a powder from Tractor Supply too which I stir into his wet food, also from TractorSupply... 4Health a very good and economical dog food, especially important with a large dog.