This article explains all the issues concerning Ethoyquin:
Problems With Artificial Preservatives in Dog Food - Whole Dog Journal Article
'If you are adamant about avoiding any amount of artificial preservative in your dog’s diet, you would be well-advised to switch to a diet that does not contain meat meals. Canned foods and frozen diets are generally made with fresh and frozen animal ingredients, which are not usually treated with preservatives. Of course, feeding your dog a well-planned home-prepared diet made of fresh ingredients is the only way to be absolutely certain of ingredient content and quality.'
As Blue Bites does not contain 'salmon
meal' that may not be the problem... but glucosamine/chondroitin is made from shellfish and could be too rich for him or he may be allergic to shellfish.