My first suggestion is to talk to his parents and find out what he's REALLY into: does he want to be a train engineer, a firefighter, a chef, a mechanic, a cop, a fighter pilot? He may enjoy the trendy stuff for a few days, but getting something he's truly interested in will be the best buy, even if it only runs ya $10 bucks :) You can really fill a gift box then!
My son has been interested in Ben10, Bakagan, Spiderman, etc., since he was about 5. He's now 6 1/2 and while he still likes them, he is now into anything Star Wars & Indiana Jones: Wii, DS (he'll be getting this for Christmas), but EXPECIALLY Legos! He's getting the Legos Darth Vader/Tie Fighter set for Christmas.
He has loved trains since he discovered the train store when he was 3 and even though he is 6 1/2, he does still enjoy his Fisher-Price GeoTrax. He can spend days designing and re-designing layouts without help from adults. They are also big and chunky and snap together and STAY together until he's ready to change them. They are just the right size for little hands and they have Remote Control (RC) trains AND planes.
My son also goes back and forth between chef, firefighter and police officer (his Daddy's profession). He has some time to decide and some jobs can be combined: Train chef, Railroad Police, Police officer/Firefighter/EMT in some departments.
Definitely ask first :)