That monk character doesn't even seem all that optimized. It sounds like he gave up a ton of power in order to grab some things that will temporarily give him higher than average AC. In a few more levels when more gold and magic items become available (Like Bracers of Armor) he will be wishing he was straight monk, as those 3 sorcerer levels will be a waste.
It seems to me like the other 4 people in the party must be just awful at character optimization, and that's why this very tamely "optimized" Monk is showing them up so badly.
I think Pathfinder might just not be the system for your group. You should look for something that is easy for your novice players to understand so they don't fall behind the other player who is actually putting forth a little effort into understanding and working with the "game" part of a RPG to make his character more powerful - which, as some other people have already brought up in previous posts, and I agree with, is debatable because this PC seems to have given up all offensive capability, and given up a good amount of future power, in order to get a high AC at level seven. I wouldn't even call that optimized, just specialized.
Not a case of being overpowered. It's a case of being meh in a team of underpowered.
These guys have nailed it. My (not entirely friendly, for which I apologize) first reaction upon reading the OP: "Imagine the chagrin if the Monk player decides to take another Sorcerer level and learn Mirror Image! Woe is me!
Somebody needs to step their game up, or play a different one in the first place, for this PC-has-high-AC 'problem' is really not that fresh..."
I'm not trying to bash on you, OP, but high AC crops up soooo often as a supposed problem when DMing any edition of D&D, one gets really tired of reading these threads over and over again. Do a search on just about any D&D board, see your topic hashed and rehashed to death in threads from 2004 or so, with much the same arguments and more or less helpful answers provided as here. But since this is obviously fresh and new to you, and I'm probably better off either helping or shutting up, here's my two cents:
I think the best advice I can give is the following. Once you understand how Pathfinder works, what's effective and what isn't, what's incentivized and what isn't, which strategies kick ass and which fall short - you'll realize that you're expecting a whole different game than what Pathfinder (or any of the more recent D&D editions) provides. Then you can decide whether the game has other merits and you want to keep playing it, or whether you want to find a different game.
So you either learn to optimize to a certain degree, you know, learn the ins and outs of the system. Never hurt a good DM to know the mechanical side of their game. To assume solid roleplaying gets detracted of by solid rollplaying is a fallacy. There's plenty of online resources if you want to go this route, and plenty of helpful folk out there to provide aid. You're forced to metagame, sure, but as a DM, that's your job anyway. You'll look back on this thread and wonder out loud: "did I really complain about a Sorcerer 3/Monk 4 being OP? Seriously?!"
Or, you can decide you really don't like how modifiers stack, or how some strategies are penalized in comparison to others even though there's no "logical" reason for this rules quirk, or how some PCs can achieve feats of specialization that run counter to your mental image of what they ought to be able to do. In other words, you can decide that certain parts inherent to the game system don't appeal to you, which, it goes without saying, is perfectly valid and fine.
But until you develop a better understanding of what's "normal" or "to be expected" in a PF game, you probably need to put your expectations of what ought to be on hold, and just observe what happens in the game. As long as the other players don't complain about your "problem" PC, and as long as you can still enjoy the game yourself, you're probably fine for a while. Try different things, observe whether your "problem" is really that (something that needs resolving), or just a quirk of that particular character (unhittability in exchange for almost complete lack of offensive power).
Above all, try not to hold the player responsible for what you might perceive as a flaw in the system (I'd call it a feature, not a bug, but YMMV) - it's not his fault.
