@Triscornia
It is also quite possible that it's simply the way his body is growing. I was 6ft tall by the time I was 14years old and as skinny as a rake. I put on a bit of weight as I got older, (you cannot see my ribs anymore) but I've always been slim.
As a younger man I tried to "bulk up" with high carb and whey protein powder drinks but they had no affect on me, - whilst my gym buddies were piling on muscle like anything, I stayed slim. There is a medical term to describe this but it escapes me at the moment.
As I mentioned above, it may be worth checking for any other medical reasons for the weight loss but it may also just be a case of being patient and waiting for the vertical growth spurt to finish to see if he puts on a bit weight.