Cash. Gifts shouldn't be about what is fun for you to buy. It's just something you think might make the other person happy. 16 year olds usually aren't sure who they are or their place in the world or what they want out of life. So if the fella says "whatever" to gifts, give currency. Everybody is happy.
Now drugs on the other hand, yeah, think I might head on down to the local meth dealers at the supermarket round the corner! Seen 'em on the bus plenty and they always seemed like good folk, sides their fucked up Lohan teeth.
Jesus once said "Give a kid some pot and he gets high for a day, teach a kid to grow pot and he gets high for a lifetime."
Some thing's just about any teenage guy would appreciate: Cash (or a visa card), a good leather wallet, and Electronics (depends on what he already has).
Depending on your family and surroundings/state, a gun might actually be a good gift. I like the swiss army knife idea.
Try to force him to find some clothes he would like under threat of you buying him clothes anyway. I'm sure he can find some. :P If he still refuses, go to threadless anyway and get some shirts you think he would like?
If he doesn't have access to weights, some decent quality dumbbells might be nice.
@panvixyl Get him Aliens HD anniversary edition (I'm sure there is one) or the director's cut of same. Maybe you can force good taste on the kid.
Not to be rude, but your brother sounds like the type of always bored kid who's doing ketamine or whippets or crunching up and snorting Adderall in his buddy's basement. Keep on eye on him, or have a heart-to-heart and ask him if he's doing "the drugs."
Comic books, maybe? I seem to remember there being an Alien vs. Predator comic book series.
I second the "something music/video game-related that's a little beyond his comfort zone" idea, too. When I heard "skater", I immediately thought ska, I know a few ska bands I like that I could recommend. Or maybe guitar tab books so he can practice?
Just give him some money. I remember when I was like 12-13 and I just started asking for and receiving money for Christmas instead of gifts my relatives thought I would like but I actually didn't want and then got zero use out of. It was great.
Since he's your brother, money might be a little bit impersonal. You could take him shopping and buy a couple things for him? I was always sad I didn't have an older sister; all my friends that did had nice relationships with theirs and I always felt like I was missing out. I'm not sure how old you are in relation to him or how your sibling relationship currently is, but later on he'll probably wish he had spent more time with you growing up. You might as well spend some time with him now before he gets too old.
I know that doesn't work for the rest of your relatives, but still, just tell them to give him money so he can buy what he actually wants.