Hi there.
It’s very possible, even likely, that this brand makes him ill – my senior cat couldn’t have Hill’s Science Diet when he was younger, as it gave him diarrhea and projectile vomiting. I had to change the brand to something higher quality for the majority of his life. Have you tried any other brands of food for him?
Hey there – because this is a potential emergency, I would call your vet. How’s he doing today?