Is he losing weight? If not I wouldn't worry too much, he'll eat when he wants. If he's losing weight and won't eat you may want to get some labs drawn and see if something's off from the blood work. My almost 12 year old Lab is apparently diabetic and pretty severely so. She pretty much quit eating for several weeks so I ended up switching her to a raw food diet and she gained 7 lbs. Took her to the vet for some other things and she popped crazy high on their sugars test as well as what their equivalent to A1C is can't remember what it's called. Vet said the kibble was probably making her feel sick because of the diabetes so she didn't want to eat it. I had coincidentally already switched to the raw food recently so vet wants to do another round of labs to see if anything changes. We were also giving her tons of snacks all the time so stopped that.