Yes, Buck is right...........especially since you've seen first hand that it isn't agreeing with him all at once.
If this is a food you want to continue giving a chance and maybe stick with, I'd mix the new food slowly in with his old food. 25% new to 75% old, then slowly move the ratio to 50/50..........75/25........and finally 100% new. Which probably means buying another bag of his OLD food, but it's better than the alternative, LOL
Imagine a toddler's tummy. Say he's only eaten strained green beans for the last year, then BOOM, mom feeds him a blendered up bean burrito or something!
The results probably aren't going to be pretty! :lol-sign: