Coming from a powergamer, I don't think you really want this. His character isn't nearly as powerful as he thinks it is. If he redesigns it, it'll probably be signifigantly better offensively, potentially enough to start causing problems. When a minmaxer designs a character that isn't as powerful as he thinks it is, my solution is usually to let him keep believing it so he doesn't go back to the drawing board and make something that actually is powerful.
What problems are caused by his high AC specifically? I realise that it means you are having a hard time hitting him, but how much of a problem is that really? He has no way of encouraging enemies to attack him I'm sure, so he's not out defendering the defenders. It's highly likely that he's not outdamaging the strikers either with that build. What you've got is a minmaxer with a build that is inneffective at anything other than survival, and has no way to hog the spotlight from other players. If it were me, I'd just let him keep thinking that he's beaten the system.