I don't have any wisdom for you - besides "go the vet" which you've done already. I am sorry this is going on and feel for you! I hope Jackson is better very soon.
My dogs both adore the "bland diet". I'm glad they can't "fake sick". They like it that much.
I take one large can of white meat chicken with the juice and mix it with two cups cooked white rice. Served warm. They have a sensitivity to chicken too - but they'll eat this ALWAYS ---- even when feeling really punk.