@Burn0
I would ask him to cancel, and your cancellation policy will be applied, and then tell him you will happily refund any nights that you sell during March.
However, personally I would gie a complete refund to anyone cancelling more than 4 weeks out. A 50% refund is fine and remember too that if he contacts AirBnB and claims under extenuating circumstances then he will get a complete refund if his claim is accepted.
Your calendar will not open up until the guest cancels - so ask them to do so immediately so that you can get the room back on the market. Remember, if you get an email asking you to cancel the room, decline it, (which means you still have the room held for the guest) as if you accept it, it will mean you initiated the cancellation.