@Yekta-Zak0
he knew he will not stay because obviously his planes have changed but he decided to check in anyway and to try to find a reason for cancellation with full refund. When he didn't find any he canceled and asked for a refund.
You have no obligation to refund him. You didn't even got your payment yet. Just tell him he will be refunded by Airbnb according to your cancellation policy .
But..
it would be good if you make photos and video of your apartment ASAP just to prove everything was ready to host him. Also, it would be good idea to contact Airbnb and tell them what happend (guest checked in, looked arround ,tried to find a reason for a refund, didn't find anything wrong , then cancelled and asked for a refund) . You can also attach your date stamped photos to prove everything was clean and ready for him.
Good luck