T.S.
Do you live near an amusement park? You could get tickets for him and a few friends and drop them off for the day.
For my daughter's 16th she went with a group of friends to Benihana for dinner. I left her with enough money to pay for it so the girls were "on their own" and they had a great time.
I see some of the other moms saying just ask him what he wants to do, but if he's anything like my son you'll probably just get a shrugging "I dunno" LOL!
Good luck :)