Do you guys both share 1 single ISP account? If you do the first thing to say is that u will install a bandwidth monitor on your machine. If you only use 500mb of a 3GB cap (for example) then you will only pay 1/6 of the ISP bill.
Then you have to say, I want to use the net so do you. We pay 50/50, but I can't game if you downloading at the same time. Therefor you are getting 100% usage. So if you don't want to compromise, slap him with the entire bill.
Once you've got him "to the table" so to speak you can discuss usage. Obviously you wont be playing games all day long. So set certain hours. Normally at best you will play from about 3pm-12pm. (9 hours) So he can download all he wants when u aren't playing.
Put it this way, if the agreement between u 2 is to share the line and u each pay 50% then each of u is entitled to atleast 50%. Any more and u r just getting lucky. If you aren't getting 50% usage technically u can take him to court. Are you at varisty? If so go there and get one of the LAW lecturers to write a letter explaining that you are entitled to your 50%. If you can't get it either he forks out the cash or the line goes. Obviously saying "I'll take you to court" is more a bluff (Unless you have the cash to take him to court) so getting someone else who is studying LAW or something to do it for you might be the best way.