It could very well be causing it. My dog has rice based kibble all his life, turns out he was intolerant.

If he has a wheat allergy and it ha wheat in, which I'm 99% sure it does. I'd be finding some different treats.

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I have Coeliacs Disease which basically a allergy to gluten found in wheat barley rye and oats. If your dog is wheat intolerant the bonio well could be your source of trouble, never mind what your vets says. It's all to do with the level of tolerance, ie I can't eat oats because I'm sensitive to them but other coeliacs can. becuse I follow a strict gluten free diet, if i accidentally eat gluten the reaction is worse as the body over reacts. I would avoid wheat products all together. you can make glutn free treats for hi using gluten free flour from the supermarkets and there are plently reciepies on line.
p.s wheat allergy/ intolerance can also manifest in skin conditions that can be vastly improve with wheat out of the diet

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"You think dogs will not be in heaven? I tell you, they will be there long before any of us."
Robert Louis Stevenson (1850-1894)

Right,will leave out the Bonio.He has Burns chicken and rice.We tried everything else when he was young and Burns was the only food that didn't upset his tummy.Will keep you updated.Thank's for the help and advice.