Tell them that your toilet is broken (or any other household "fix" excuse you like, plumbing is good) and you are so sorry but they cannot stay there. Offer to give them 50 bucks towards a hotel. Any story you make up will work, any time it is "fixed" or "still isn't" is totally viable. Do it and don't look back.

Would be cheaper and less stressful to just pay for him to stay in a cheap hotel. This is a no go for me :)