@Suy62 An inquiry expires after 24 hours. I shouldn't think that this guest would be able to book at this point. Have you looked at your hosting pages to see what this inquiry says next to it? It should say "Not possible", since the guest didn't book it within 24 hours. If it doesn't, I'd contact Airbnb to make sure he can't still book.
Have you tried messaging the guest to find out what his intentions are? That's where I'd start.
It's more common than not to never hear back from Inquiries. They may have sent out inquiries to 10 different hosts.