He could be allergic to the salmon/mackerel and instinctively avoiding
it. It's usually not the fish they're allergic to but the perservative they
use to keep the fish from going rancid, very common issue. Take him
off kibble and feed old kibble (did he have issue w/chicken) or feed a
bland diet for a couple weeks of boiled hamburger (drain fat) & white
rice then add new kibble. If he starts throwing up, it's the new kibble
causing the problem. Hoping you find the cause asap!