@Tracy784 Just tell him that you might be able to rebook the dates and would refund him something if you can, but that he has to officially cancel for the dates to open back up. If he doesn't care about a refund, that's even better- you've gotten paid for dates where you don't have anyone using up utilities and amenities, dirtying towels and sheets- lucky you.
Do make sure to have that conversation on the Airbnb messaging system, not in person or by phone. You want a record of him saying he doesn't expect any refund. He sounds like a reasonable guy, but you never know if a guest is going to change their mind and try to get Airbnb to refund them for some bogus reason.