I doubt he is seeding torrents for fun. He may be a member of private trackers that require him to maintain a certain ratio.
No one else seems to be saying it, but I'll reiterate. Get him a seedbox. He benefits (faster downloads, more upload bandwidth, greater security and no potential problems with his ISP), and you benefit (all he will be using the Internet connection for is downloading files from the seedbox, which is nowhere near as bad as torrenting on the Internet connection itself)
He benefits, you benefit, everyone benefits. At ~£10/month it is presumably less expensive even than paying your fair share for the Internet connection.