Can an employer cut the commission rate at will?

If there is an employment agreement of some kind, including a letter from your son's manager or the store, setting out the commissions rate, then the store would have to honor the terms of the agreement. For example, if it said that for 2009, commissions are X%, the store cannot change them for one day. However, in the absence of an agreement, then businesses may change commission rates at will--not for sales already made, but for forward-looking days or sales, as this seems to be. So it does seem to be legal.