Welcome to you and to your dear companion JaBok.
First off, I would take JaBok to the vet for a complete check up - when was his last one? Was that 3 years ago? With this new problem, I would get him in a.s.a.p.
After you mixed the holistic food with the chicken oat porridge, you mixed it with your regular kibble. He had trouble with the combo both times. Could there be a problem with the holistic food?
Your comment about JaBok makes me think of a post on here about pre-mix (do a search on 'pre-mix' in the search area of this forum). The post was about some tainted dog food (kibble). A 'pre-mix' (many of them are made in China) which is added to the kibble ingredients to provide proper vitamins and nutrients was making dogs sick. Owners were recalling their dogs turning their noses up at their food.
In any event, I would not guess.I would get him to the vet and in the meantime, feed him boiled chicken with boiled rice (a tiny bit of chicken stock for a little flavour). Try to bring along a poop sample. If he vomits anything, bring that along as well. Please let us know how things progress. Wishing for the best.