Fifteen-year-old boys are sometimes hard to buy for, but if you take into account their personalities and hobbies you will have a much easier time. At 15, they still can't drive but they like to hang out with their friends. There are many things you can get them that coincide with their "likes" that they will love.
A flip cam is an inexpensive, compact video camera that he can use to record his friends. He can use it to record fun times spent skateboarding or doing other sports. He can edit the video and show off his skills. You can get a custom picture put on the back of the camera or pick from ready-made designs.
Get him the big, powerful ones or ones with suction cups. Make sure to buy a target as well, otherwise he may end up shooting at things you don't want him to hit.
A roll-up piano is a keyboard that plays like a standard keyboard but can be rolled up and taken with him or stored. This is a good gift if he has an interest in music and being in a band.
Stilts and Pogo Stick
These are fun, retro gifts that he will love. He may be the only kid on the block with them. They will keep him active and outside playing. They are also something he can enjoy with friends.
If your 15-year-old plays a lot of video games, get him a bean bag chair with surround sound. This way he will not have to turn the TV up loud when he is playing. Also, it provides him with a more complete gaming experience.
Noise-cancelling headphones are a great gift for the 15-year-old who listens to a lot of music. Skull Candy or Beats by Dr. Dre are great choices. These are large headphones that block out all other noise and let the listener hear how music sounds when it is played in a studio.