AskNott, I now know way more about you than I ever wanted to. The mental image I have right now should be grounds for a lawsuit.
(shudder)
It might sound like a crazy idea, but you could ask the parents what he’s into these days. Or, better yet, if he’s not an only child(besides you, that is), ask a sibling who lives with him, they’re a great source of info on that sort of thing; you make hear complains, but you should get some ideas 
What’s wrong with porn? He’s a man now, right ;j ?
Gadgets; any kind of gadgets.
How about a pre-paid mobile (cell) phone outfit? He can program some sort of particularly obnoxious ring tone into it to annoy yer step mum, and she’ll have to cough up the money to help him pay for future sim cards, or whatever they’re called on your side of the Pacific.
OH GOD, after my recent rant about mobile-phone users, please excuse my blatant incongruity in suggesting such a gift, OK? 
Wow, some of you are really out of touch.
Army Men? Risk? Sock-Em Boppers? (although the alternative suggestion for their use is something he’d appreciate)
The Kid is 13, you know, high-school next year? Not 8.
I would have to go with the suggestion of Gadgets. Something really slick from the Sharper Image or some similar store.
But definately don’t buy any obvious “kids” toys. He’s 13 right, he’s had his Barmitzfah? He’s a man. Not a boy.
A subscription to Mad Magazine.
Drums?
Amatuers.
Bagpipes!
And a subscription to Maxim.
Maxim is a great idea. Enough of what he wants without being embarrassingly obvious. Maybe a swimsuit calendar too.
Anything with Blue LEDs in it.
Binoculars. Really cool small but powerful ones.
The kid lives in NYC right? Could be as good as porn.
Either that or cash. (in small unmarked bills)
Or (to please kid and annoy step mom) some of your underpants.
let the ducking and running commence!
Magic: The Gathering. Find out from your parents if he’s already playing. If so: Go to your local game store and buy some high-end decks. If not: Go to your local game store and buy some basic decks.
Boom box (or a really good pair of headphones for the one he’s already got).
Get him a hop-on.com disposable cell phone.
What the hell is a Toast Launcher?
Um. . . ew. It’s her BROTHER. Hello?
When I was 13 I killed for books. When my middle brother was 13 he killed for a skateboard. When my youngest brother was 13 he killed for a push-scooter (and this was like 10 years ago, way before Razors ever came out).
Tell the kid “I’m gonna get you 13 pairs of argyle socks unless you give me an idea.”
This is really a great idea, but as a 13 year old he would probably appreciate it if you spent the extra cash and threw in the x-ray filter as well! 
HALF BROTHER.
Still, sending him my undies would be more than a little icky and Freudian.
Thanks for the ideas, guys! I’ve done baseball/software/Harry Potter stuff lots of times before, so I wanted to try something new. You’re given me some good fodder. I think I’ll skip the bagpipe, though; the neighbors might kill me the next time I visit.
I’ve had to go to the gift cards for my nephew, who is 13.
What does HE like? Television. and that’s it. My sister has allowed him to grow up with an unchecked diet of pure television. Quality doesn’t make any difference, it’s all quantity. If it’s on TV, he’ll watch it, and he’s the stereotypical zombie watcher, oblivious to everything but the call of the box. I used to get him toys, books, games, all kinds of stuff, but none of it took. I was amazed a couple of years ago when I went in his room and looked around…* there were no toys at all*. Nothing.
He can watch TV for hours on his own, but nothing else. If he doesn’t have TV, someone else must amuse him - he’s incapable of entertaining himself, demanding constant external stimulus. He has no imagination as well. Despite him being able to make the usual teenage sarcastic comments, he absolutely can’t detect sarcasm when it’s thrown back at him, taking everything at a base, literal level. He briefly showed an interest in Harry Potter, the only books they’ve gotten him to read, but didn’t like the first movie because it wasn’t exactly like the book word for word, and he didn’t understand why it wasn’t.
Sorry to digress there. What I’m saying is, he may as well be from Jupiter as he’s so different from me at that age. So I just aim a gift card at him and let him go to town on whatever it is he likes.
**Lego ** you have just made me shudder for that poor child.
And made me turn off the idiot box down stairs.
That is just plain sad, sad, sad.