It seems to me the biggest bang for the buck for an Avenger is just taking leather armor. +2 AC (at least), one less feat, and no stat pre-reqs. The OP's player missed a trick, methinks.
I expect it will take about 2-3 encounters of being pummeled with 7's or even lower on the die to want to rework his character.
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.
....Yeah......The player in question likes to hog the spotlight as much as possible....
..... I just need to knock him down a peg or two is all. Problem is if I do that he'll feel inferior again and probably want to redesign.
The player in question likes to hog the spotlight as much as possible. If I only use monsters that attack AC then he gets what he wants and I get 3 party members laying on the ground because I focused fire on them.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.