@Christopher187
as far as I know, you can send him an alteration request where you will shorten his stay from let's say 6 nights to just 1 night (if he just arrived today). He has to agree on it. Then he has to book your other room. He will be refunded for the first room and will be charged for another room and your calendars will be updated - the first room will be available and another room will be shown as booked.
If he can't book another room for today or tomorrow then look at your availability settings/advance notice. Maybe it is set to 24 or more hours in advance, you should change it to "none". If you already have it set to "none" then maybe it is impossible to book more then 1 unit for the same dates (I am not sure)