I guess it's a very difficult decision to make. I would imagine that a 16 year old would have a pretty good idea anyway...
My nan died from stomach cancer about 10 years ago. She was diagnosed with terminal cancer and recommended for an operation - her children took the decision not to tell her about the terminal bit, on the basis that she would 'give up' if she knew. The op was supposed to give her an extra 6 months to live and in reality she died pretty much 6 months to the day. At the time, I agreed with their decision but now I think I would rather have been told.
Another situation - one of my customers has cancer of the nose and because of where it is, it is obvious that something isn't right. Although she is really upset, she is glad that she has been told exactly what it is and what the outlook is so that she can enjoy the time she has left - take holidays, spend all her money and not worry etc. At first, she was told she had polyps which could be removed.
I think it is a decision that is probably different for everyone, depending on circumstances. Personally I would rather be told so that I could do things now that maybe I wouldn't do if I thought I had plenty more years to do them in. Even for a 16 year old, he could have told his family how much he loved them, said goodbye to his friedns, planned what he wanted at his funeral etc...
I guess that what I am trying to say is that I think he should have been told the truth.