I have 3 teen DS - ages 13, 15 and 19 - it sure is tough to find things for them. Besides candy and gum here's what I have:
DS13: Scrubs season 1 DVD, 2 DVDs (music), 2 CDs, 1 Best Buy GC, 1 T-shirt (UFC)
DS15: Scrubs season 2 DVD, 2 music DVDs, 2 CDs, 1 Best Buy GC, 1 T-shirt (Under Armor)
DS19: Will probably give him cash - about the total of what the other boys are getting. He's a poor college student now, so cash is best.
They've all just had birthdays recently too, which makes it even harder (Feb 28, March 6 and March 19). It was so much easier when I could buy toys for the sandbox!
I'd recommend gift cards - sports store, electronics, movie theater, etc. Maybe a new T-shirt (I rarely buy DS15 Under Armor for no reason since it's so expensive, so he'll be thrilled!). The video games they're into now are each $60, so I figured they could put the Best Buy GC toward one if they want. Good luck!