My flatmate wont do anything. Help?

I have lived in a flat with three other people for the past four months. For the first month we just bummed around and didn't really clean but as the house got messier and messier we all sat down to have a flat meeting. We came up with a schedule that everyone agreed to where we have one job that we have to do a week. Cleaning the bathroom, cleaning the kitchen, putting out the rubbish and vaming. So far he has put the rubbish out twice and vaed once. He has the biggest pile of dishes in his room and he just wont clean up after himself in the kitchen. We have asked him nicely to do his jobs and complained to him about the mess but he just says "uhmm" and never does it. My other flat mates also hate him not doing anything but we are at a loss for what to do next. I hate having to nag him as he is a good friend but I'm going crazy. I know it shouldn't bother me as much but I really need help. What should I do next?!?!