There are several possibilities for thinning hair or hair loss in a teenage male.
It can be hereditary, for example. But it can also be due to hypothyroidism. Are there other symptoms, such as intolerance to cold, weight gain, and sleeping too much? It's important to tell your physician about any of these symptoms along with the hair loss. Hypothyroidism can be life-threatening.
Otherwise, how is your diet? Are you eating a balanced diet with healthy sources of protein and fat? How are you managing stress?
The Merck Manual lists other causes of hair loss:
http://www.merck.com/mmhe/sec18/ch207/ch207c.html