Others have already mentioned that the rules limit you to two magical rings and that named bonuses don't stack but there may be other problems with the sudden rules clarification. If you've been letting the player get away with it for a while and he's invested a fair amount of money in magical rings of protection he might feel hard done by if he suddenly has a bunch of rings he can't use and wouldn't have bought if he'd known the rules at the time. You can do various things to alleviate the pain here.
Firstly, it's fairly simple to either let him convert the rings into a more powerful ring. You'll need to look at the cost of the rings to work this out but I think, off the top of my head, a Ring of Protection +1 cost 1,000gp and the +2 ring costs 4,000gp. He's not going to have as high an AC but he could trade four +1 rings for a +2 ring as if he's keen to keep his AC high he'd probably have upgraded if he'd know they didn't stack.
Secondly, if you want to bring this in to the game more and explain it through the story you could have the rings he's wearing start to cause problems as they interfere with each other. Maybe his AC starts getting a bit random, sometimes being higher than normal, sometimes penalising him. Alternatively the deflection aura could be greatly magnified, raising his AC by a ludicrous amount (+10 or so would probably work) but preventing him from getting anything close to him. He wouldn't be able to eat or drink with the rings on and, if he persists in wearing the rings he may find himself unable to breathe. This should encourage him to remove the items and look in to why his rings are having such problems leading right in to an adventure to find the old wizard Crimlock, an expert in magical item interference. Of course, he most likely lives far away from civilisation as the interference may cause problems. In fact, the players may have to save him from some problem of his own creation before he can help them by combining the rings in to the single, less powerful but more stable, Ring of Protection +2.
Depending on the player you may wish to talk to him about this before you start the quest line explaining why you are doing it. He may like the idea or he may prefer to just swap out the rings out of game and hand wave the mistake away. Either way works as long as everyone is happy and the game continues.