@Josephine10
First step is your guest has to cancel his booking. Make sure he does this immediately- and that he does not ask you to do so. Under no circumstances should you initiate or accept a request from him for a cancellation.
your cancellation policy will then come into play. Remember that you have received no money to date so you have nothing to refund. You will be paid according to your cancellation policy approx 24 hours after your guest should have checked in.
you can tell your guest that the sooner he cancels the more opportunity you have to resell night (which if you do, you can offer to refund).
Note that if he wishes to claim under AirBnB's extenuating circumstances policy then he may get a full refund. He will need to provide certain information nation to prove his case.