First, if he's a fifth level fighter, he only gets on attack per round. Did you mean sixth level or higher?
SEcond, if you house rule this, I highly recommend telling the players about the new house rule before the session begins, so they can take advantage of this new tactical option as well.
The normal tradeoff is that readying an action disallows the use of a full-round action: you can do less stuff, because you take the time to ready the action (i.e., getting in position and watching for the action's trigger). If you're going to make readying an action as easy as any other action, let the players know -- after all, the PC might decide to ready an action to trip the fighter when he attacks, and then follow up with the remainder of her attacks.
Third, I recommend against this house rule. Readied actions are already confusing enough, and this is going to make it much worse. If a fighter gets three attacks in a round, and she readies her second attack to disrupt a mage's spell, when does her third attack go off? immediately after their second, readied one, or after the mage's turn? If it goes off after the second attack, does damage from it force a concentration check for the mage? Can the mage take a 5' step between the 2nd and 3rd attack? Next round, how do the mage and the fighter compare in initiative order? Can a hasted character cast a full-round spell as a readied action?
There's a way to accomplish what you want within the rules. Actually, there are two ways:
1) Have the fighter hasted. He attacks as a full-round action, and then with his extra partial action readies the sundering attempt.
2) Have the fighter attack normally during the first round. On the second round of combat, he does the readying thing.
Daniel