Walmart, Sears or Penney's.
Walmart is probably the cheapest, but the quality might not be the best either.That said, if he's growing quickly, or is hard on them, that's ok, you'll need to swap them out more often, so no point buying expensive ones anyway!
Penney's quality is just above WallyWorld, and Sears above that, for about the same price. Better if you hit a sale. My DS (18) still has a pair of jeans we got him at Sears a few years ago and they look great!