I like to use hydrogen peroxide. it bubbles so the dogs don't like it close to the eye, and be careful to not get it in the eye
but I soak a paper towel and squeeze out a bit on the top of the stain, and rub. it works well.
but I think you will find that the food he is eating will also cause this, the darker meats, lamb, beef,they can contribute to the stain, try chicken,fish, try Fromm food, I haven't heard anyone say that food is causing any stains, and it may be that your dog is just not agreeing with what you are feeding. but it is probably the protein is a bit wrong for your dogs system that's all. good luck, many people who have white faced dogs , just live with it. good luck, and happy Thanksgiving.