long reply is looooong
I don't think Pokémon cards are "in" anymore, but if he's a Pokémon fan, chances are he'd enjoy them. I think booster packs (with 11 or so cards) run about $7 each these days. You can also get tins with a lot more but I don't know how much those are.
As everyone else has said, there are lots of figures or plush toys available. Depending where you look (Toys'R'Us has a great selection), there are even playsets that the figures can go in and whatnot, so that's probably a good idea too. They're fairly decent quality and, for the figures at leat, there are usually a lot of them available at any given time and not just the "bad" Pokémon that no one likes (as is the situation with plushies here).
Another gift idea, if the child has access to a DVD player, would be DVDs of either the movies or the TV show itself. They have the season boxsets for about $30 with 30 episodes or so on each and the movies are always good and retail around $20 each.
As everyone else said, the "main thing" right now is the video games, especially the ones on Nintendo DS. So if that's an option, I would suggest the DS and Pokémon Diamond or Pearl, although other games like Mystery Dungeon Exploreres of Time/Darkness or the Pokémon Ranger games are decent buys too. You may even want to look into getting a used Gameboy Advance with some of the older games for that system. Shouldn't be as pricey as a DS system and the Pokémon games for that console are still a good play.
Hopefully that helps. I'm not sure what your price range is, but the Pokémon series is something that can be enjoyed in many mediums, so it shouldn't be too hard to find a suitable gift. :)