good for him... you should definitely encourage it. I would however recommend that you do a little bit of reading on here and tell him to do the same.
If he is showing a serious interest in it 6 months from now, encourage him to look into the Air Force or Naval Academies. He will have to really start focusing on his grades in high school and playing a sport also helps out (anything extra-curricular is good though ie: flying). My buddy is now flying F-16's and he just turned 24. He went to the AF academy and all of his training was paid for. He has a fulfilling/stable career as a military aviator to look forward to and a "foot in the door" when he decides to get out or retire and go civilian. It is the best way to go these days.