What kind of bones are you giving him? Sounds like the big beef marrow bones. I don't give those to my dogs at all anymore, those are the ones that can chip their teeth, and don't do much good as far as removing build-up (IME). Letting him crunch up more manageable-sized bones whole will clean his teeth thoroughly and give him the nutrients from marrow, and calcium and phosphorus from the bones themselves. I feed my dogs and cats mostly poultry necks for teeth-cleaning. None of them have any tartar, they are 14, 10, 5 and 3.