You say no one is qualified to make that call and then hint that he might have an eating disorder and that he should see a doctor.
In my opinion seeing a doctor is a massive over reaction and I agree with DLS 100% having gone though the same thing myself and now back to being a bit of a fatty.
Not only have I gone throught this but I have seen this occur in many other 16-17 yr olds. Basically the story goes like this, teenagers are fucking mean as hell and will tease the shit out of any fat teenager, fat teenager gets sick of being called fat and when you are 16/17 is probably the first chance you have to hit the gym, go running and be independent and make decisions on what you will and won't eat. This leads to under eating, too much cario and massive weight loss and usually the kid is afraid to eat as they don't want to go back to being the fat kid they once were. Also chances are a lot of people have been saying how good the kid looks and admiring thier ability to loose weight which just reinforces the bad habits that they have developed.
Finally I don't think being 60kgs @ 6 foot is that bad, a little underweight yes, but not life threatening. Your body is designed to go for long periods with little food, that's why it is so good at holding onto the fat.