How about walkie talkies as a gift? Now I know that's got nothing to do with exercise but it could be turned into a game!
If he gets some friends together to play a treasure hunt or a race (you see all the TV shows of games where teams have to overcome certain obstacles) and get to a finish line? Well kids are imaginative and I am sure he could think up some games to play that involve tracking the competing team or sending instructions to a fellow team-mate if they are doing tasks in a relay-style game. With all this running around and the excitement of a win, he would not feel he is exercising and the fact that he owns these cool walkie talkies means he is always the first picked for the game.
Walkie Talkies can be inexpensive - you should shop around and see what is out there, like this from
Amazon.com