Honestly, @Janet129, if he forgot to cancel, this is more his problem than yours. I don't mean to sound hardhearted, but it was a pretty easy thing for him to cancel, and he didn't.
If you are determined to reward him for his indolence, you can go to the reservation (under Your Reservations) and follow any prompts there. Change Reservation gives you a price option. Try that. Has the checkout time passed? It may be too late for that then. Someone may have experience with the Resolution Centre for something like this, but it doesn't sound as if it is worth your time and trouble.
I did have something similar happen, not quite the same, but I offered to reduce the guest's price on his next booking with me through a special offer, should he book again. You could do that?