You could try a little homemade chicken broth, nothing added (not the store bought crap). He either got into something or picked up a virus. IMO (I am not a vet)
When he is on his feet you may want to consider whatever additional testing your vet recommends. I know, ($) but these things happen and once they stabilize you may get lucky and get a 5 year run with him needing nothing but an annual/routine care/heartworm check etc.
I recommend Nutrisca dry, soaked in water for a dog with a sensitive stomach, just tried Orijen, it’s quality stuff but I think it might be too rich for 2 out of 3 of my dogs. I will continue to use it however as a base, maybe I’ll mix it with the Nutrisca.
