It could be, he may be downloading big files, music, anything, that will eat up your bandwidith.
Does anyone know how they stand if the other person is downloading illegal material?
Personally, I would disconnect him, fix the IP addresses to your MAC address and reduce the number of ports available on your router. If its only you in your home, then I would restrict it and lock it down.
If its not 'big downloads' it may be a good thing to check your connection speed here
Broadband Speed Checker - THE UK's No.1 Broadband Speed Test