I find it really hard to find a grain free food that does not have something like oats in place of rice though.

I looked at Acana Singles with single protein source and it has oats in even though claims to be grain free.

Barking Heads Fusspot 'salmon & potato' has oats in too.

Jame Wellbeloved has a grain free Lamb & Veg but looking at ingredients the first ingredient is Turkey!

Bob & Lush was best one I found with single source of (novel) protein for him which he hadn't had before with well explained ingredients on their page and NO grains at all.