Agree with that! Make it clear what you expect and what will happen if not...then if he doesn't do it, you just leave his clothes in the basket (or floor!), don't wash up his things, don't pay his bills, post his letters, make his meals, or anything else you wish to withold!
As for him saying you only have that house because you work 5 days a week - that makes no sense at all!!? If he agrees that you are working 5 days a week how the hell does he expect you to keep house at the same time? How many pairs of hands/eyes does he think you have FFS?
Why not try making a rota - stick it up somewhere eg fridge, detailing the whole week. Block off - use colour or something visually clear - all the time you are both at work eg Mon-Fri, 8-6. Then split up the chores on the remaining times, and allocate it 50/50. Do one for each week, and tick off each allocation as it's done by either of you. When it's not done, highlight it. Once he has done several days'-worth of arse-sitting, show it to him and ask him to complete it.
Make sure he knows that your mindee's mum was horrified - ensure he knows if you lose business then you won't be able to afford to live there. If he says that you need your income to live there, maybe this will help him to see what he is risking by not sharing.
Oh, and make sure he understands that he is not HELPING - he is SHARING. It is not YOUR housework.