Bad online service

I made an online purchase on sunday (nike airmax 90)... Payment was made and i got a notification for order p690983.... that payment was successful and that my order was being Prepared and they will let me know once it's on its way. Told my son about it and he was all excited.... I just got a notification now that my order was canceled because they could not source the item. How can you take payment for an item and tell a customer that the product is being Prepared and 2 days later just cancel the transaction... What do i tell my son now?