In general, a few things that can play a role in a child’s weight are growth spurts, activity level, and genetic factors. Having a lower body weight due to one or a combination of these things could be considered normal (for example: he is at a point where he is growing very quickly, while at the same time playing sports, and the men in your family were thin kids as well).
However, there are other things that may be occurring or could also be playing a role. Is he eating a healthy diet? If there is a consistent calorie deficit (that is to say, he consistently consumes fewer calories than are burned) you could expect a decrease in body weight. If he is eating properly and eating enough, is his weight fairly stable, or does his weight continue to drop? If so, it could be time to visit the family physician in order to see if there is something else going on.
Putting any normal factor aside, since his weight has you alarmed the best place to start is a visit to his pediatrician. They can best help you determine if this is something temporary, or if there is any problem. If there’s nothing wrong, the visit will put your mind at ease. If there’s something else happening, they can help make it right.