I have a Kalashtar Pacifist Cleric, in the group I DM, that gives me similar issues. If there's something that will boost his passive perception, he's got it. I refuse to be stopped by him.
Even if you notice a monster with Perception, you still have to defeat it. By itself, it's hardly game-breaking.
Now, some groups are very good at changing the odds of a battle in their favor, by scouting, setting traps, and preparing buffs... It's a question of play style. It's not wrong, it's just a bit different from the heroic kick-in-the-door style of 4E.
The player of the Kalashtar chose to put his resources into Perception, so don't make it a problem, roll with it. In fact, reward his choice by preparing information that he can gather through his excellent perception check. Prepare extra-difficult encounters, that the party can beat by good preparation. Ideally, build the tools to beat the encounter right into the encounter.
(If that means they get more XP and advance faster and this becomes problematic, just dial back total XP...)
Example: If they fight a lich, drop a few hints on how to identify and destroy the phylactery, which should weaken him and allow the party to defeat him.
Or: Place some fire elementals in a tunnel that can be flooded.
You don't have to do this all the time, just often enough that your players will start looking out for these things. You're the DM, you have more control over the game then you think.