Easy. Tell him that he needs to start paying for his share of the Internet bill. He also needs to respect you and your needs. Really, if he's not paying for the Internet, he has absolutely NO room to argue and needs to free up YOUR bandwidth. Make sure he knows of your situation and if he doesn't comply, then pushing is coming to shoving.
You can download NetLimiter (
http://www.netlimiter.com/download.php) and use that to limit his upstream bandwidth (it's the fact that Skype is using every ounce of upstream bandwidth that's causing the issue) to 50% of the total upstream bandwidth. It would have to be installed on his computer, though, as far as I know.
Push comes to shove, if he's using wireless, enable MAC Filtering and block his MAC address.