A bit more info is needed. Is he not eating his pellets at all? Is it a new bag? or the bottom bit of an old bag? How is his poops looking? Is he chewing on toys ok? How much hay is he eating? How is his weight, any weight loss?
It can sometimes happen that you get an off bag of pellets for whatever reason, or the bottom bit in a bag can get a bit stale and not taste as good. Even if your other chins are eating the same food fine sometimes one may be more picky. You don't want a bag of pellets open for more then a few weeks to a month. If he is just eating less, an adult they only eat about 2 tb of food a day, some do eat a bit less, even if free fed unlimited that is roughly how much they eat, the rest of the diet is hay. Also if you got a new bag/bale of hay or something different (different type, brand, etc) sometimes that can increase their hay consumption if they really like it, at least until it's no longer "something new". Eating mostly hay is not a problem for an adult.
Since he is still eating hay and drinking water, and everything else is normal, I wouldn't worry too much just yet. Adult chins can survive (but NOT thrive) on hay alone assuming it's good quality, pellets mostly just make up for any lack of nutrients in the hay and make sure they are consistently getting proper nutrition. If he isn't eating any pellets at all though, even after trying a new bag if that might be the issues, or if the his poop doesn't looks right, he starts losing weight, or not acting normal you will want to find a vet that can see him.