will this work for you? Along with the b12?For now until further tests done I would personally feed with the highest calorie wet foods you find that DOESN'T have what you've been feeding him in the ingredients, at least not as one of the firsts. (Like if he's been eating chicken-based food, switch to a different meat.) And free feed or hand feed with some grain-free kibbles - mostly because we can't leave wet food outside.
How are things?