How to cap bandwidth on a network?

Hi, I'm new with all of this stuff, and I was wondering if it was possible to put a cap on how much of the overall net, other people on the network here in my house can use.. My son bought a computer and set us up on a router, but ever since then, all I've ever experienced is lag. Pure lag. I want to basically throttle his bandwidth so that he just has enough to get by. Since all he's ever doing is using that fucking Bit-Torrent, and playing online games, like the selfish piece of shit he is.

Any clues, ideas, help?

Also depending on your model of router you may be able to flash it with open source router firmware (read: better supported in general, more secure and better programmed).