I just had a guest reserve a room in my house yesterday, but now I have to cancel. What do I do and how do I guarantee that he gets all of his money back?
I'd suggest contacting AirBnB ASAP and explaining why you need to cancel. Normally there are severe penalties for a host cancellation.
What is your cancellation policy? Would you be able to ask the guest to cancel? If he's refunded in full, you could make a private arrangement with him so that he doesn't lose the AirBnB service fee that he paid.
I hope someone with more experience will come by to advise you.