A good way to tell if they are underweight is to feel the ribs, you should be able to easily feel the ribs under a thin layer of fat. If it feels more like just furry bones he is likely too thin. It's also a good idea to keep a weight record on him, weight loss is often the first, and in some cases only, sign something is wrong.
If he is thin you can first look at making sure you are giving him enough food, although measured is recommended restricting pellets is not needed unless the chin is fat or eating a lot of pellets but ignoring the hay. You can also look at what kind of food you are feeding, there are a lot of different foods on the market but only a few are really any good for chins. If he is getting enough good quality food then a vet check would be a good idea to find out why he is thin, chins should be able to maintain good body weight on a diet of good quality pellets and hay alone.
