When I was a young lad probably around that age I had the opportunity to sit in first, there weren't enough seats available for the family and we had to book one of them in first and for some reason they let me sit there on my own. They made sure that I understood that it was a big deal though! I still remember that flight fondly, the FAs seemed to enjoy treating me well and my biggest memory was the welcome drink of orange juice. I don't remember much else about the service but I remember that welcome drink vividly! Also in the future if people asked if I had ever flown first class before I could say that I had, albeit at a young age.

It'll probably be an experience he'll never forget and as long as you explain that it is a real special thing then I'm sure he'll understand that it's not to be expected in the future.