When is an underage person "ready" to wear thongs? This is always a delicate topic.
I'm posting because I remember how much I wanted to wear skimpy underwear when I was 14. My parents did not want me to wear skimpy underwear at all, and would not buy them for me. Still, in secret, I made my own thongs and skimpy shorts by cutting up old briefs and boxers.
All in all, I think that if your son wants to wear thongs and he is able to do it responsibly (in private), there would be no problems. Giving him thongs for his birthday, since he seems to want them so much, is a nice thought. I'm a little bit envious. I would avoid throwing away all his boxers though, starting slowly is fine.