Depends on what you want to do.
He can send you a request to alter the booking which you can either accept or decline.
If you decide to decline the change, how much he would be charged would depend on the cancellation policy you have set.
If he cancels the booking, then you can tell him that if you manage to rebook the current dates then you can offer him a partial refund, so the sooner he cancels the better the chance of you rebooking.
You can't have received payment if he hasn't actually stayed with you as yet.
If you accept this suggested alteration and nearer the time he wants to change again you will find yourself in a situation where you keep losing out on bookings.