I used to have ADS L620s and I would disagree with the suggestion that they would be appropriate for a teenager. They certainly weren't the last word on refinement, but they were a little more toward that end of the scale than the Klipsch/Cerwin Vega/JBL suggestions above. My advice -- take the kid to a store that has the boomy/sizzling speakers described above as well as something approaching more musical neutrality and see what moves him more. If his taste is pretty much all rock and he's like most teenagers, he's gonna love the stuff we've all outgrown.
Used speakers
I've recently upgraded and am giving my nephew my ancient Hafler 220A amp and Hafler pre-amp. The amp is rated at 120wpc. As befits a 16 year-old, he likes his music loud with lots of bass. I'd like to get him some used speakers in the $250 range. Any suggestions, biggest bang for the buck? Do I need to spend more?