Cooling off periods for credit agreements only apply when they're signed away from the trader's normal place of business, which would not seem to apply in this situation.
Further (when purchasing from a store, rather than online) there is no cooling off period in respect of a contract to purchase (which is separate to the credit agreement). Even if DFS permitted the cancellation of the credit agreement, your son would still obliged to meet the separate contract for the purchase of the sofa. (i.e. DFS could insists that he paid cash or, only if they were agreeable, enter into a different credit agreement).
Chris