My BIL stayed with us for a couple months a earlier this year when he was kicked out of his place. We actually did not charge rent, but he did have to help pay for gas (we were driving him back and forth to work and anywhere else he needed to go since he lost his license). He also bought some of his own groceries, although he still ate ours also (then again, when he got himself something good I helped myself here and there teehee ). I think as long as it's on a short term basis there's no need to charge rent. However, I do understand your point, if you charge him nothing then he may have no incentive to leave. My BIL's incentive to leave was his pet snake that he was not allowed to keep at our house. The snake had to stay with a friend, and he missed his snake, so he did work at finding his own place.