I did my first flight alone when I was 6 years old (although with assistance from both airline and airport personnel) and my first totally independent flight when I was 12. Around the age of 15-16 my parents allowed me to fly alone internationally and at the age of 16 I went all by myself to France to attend a one month French summer course at a university in western France (the host family I stayed with was really impressed..).

Except for the administrative parts (like a letter he's allowed to travel) I think it depends a lot on how mature your son is. Is he very independent/responsible for his age? Has he travelled a lot before? Has he travelled anything alone before (locally or domestically for example)?
If you can say yes to these questions or most of these questions I really think there is no reason not to let him fly.

My parents let me fly alone at a very early age because they knew I could handle it, they knew I was a responsible person and that I knew what to think of and to look out for. I had also travelled a lot with my dad as a child so there really weren't afraid at all to let me go, and so far I have not had one single really bad travel experience.