I started off pretty sympathetic to the player, until I found out he wanted to join the Prestige Class "Order of the Plot Breakers." That's just silly! In fact, a prestige class like the one you described would ruin roleplay opportunities in many campaigns (mine included); instead of having to roleplay in order to learn the secrets of a city, you just use a class power. No fun at all.
There's another method you could consider, one that might eliminate conflict:
-Tell people, "If you want to join a prestige class, talk to me about it, and tell me what you want to do, and I'll try to accommodate you."
-When he tells you what he wants to do, tell him you'll try to incorporate it into the setting, but that the PrC to which he'll eventually gain access may have significantly different powers from the ones he described.
-When you can, introduce a class with interesting and fun powers that don't break the setting. Consider giving a social-oriented character powers like:
* Creating a sanctuary-like effect on demand.
* Casting Calm Emotions 1/day (language dependant, mind affecting)
* Cast Emotion 1/day (see above)
* By spending 10 minutes talking with someone, make a bluff check opposed by their sense motive, in order to get them to let a specific secret slip. Succeed by 10 or more, and they don't even realize what they let slip; succeed by 20 or more, and they'll steadfastly deny that they ever let it slip, even when confronted with evidence to the contrary. Serious modifiers may apply to the role: getting someone to let slip the name of her childhood crush might entail a +5 bonus on the bluff check, whereas getting a trained assassin to let slip the name of his employer might entail a -10 or greater penalty on the roll.
* Any staff/henchmen/followers are fanatically devoted to the character, as long as the character pays them some attention (either by treating them well or by playing them off against one another).
In other words, I'd give the player some unique and fun abilities to use, some stuff that'll make him feel more effective in social situations than his companions. But I wouldn't give him things that would break the plot down entirely.
Daniel