if you choose California, head for Anaheim for a few days so he can "do" DIsney and Universal Studios[ even better for teens] from Anaheim you can even do a day trip to Tjuana and then head off for a beach area.best of both worlds.
You can do the same in Florida- week in Orlando and a week at St Petersburgh beach.
What about San Francisco - great city for walking around, riding the cable cars. Take a boat trip to Alcatraz prison, go to Muir woods to see the giant redwood trees. Take an overnight trip to Yosemite national park.