First off, I'd ask him what aspect of his character concept makes him a paladin, since there's nothing paladin-y about the way he is acting. Quoting from the description of Paladin in the PHB: "To you is given the responsibility to unflinchingly stand before an enemy's charge, smiting them with your sword while protecting your allies with your sacrifice" (P89).
Religion isn't the exclusive domain of the cleric/paladin.
Next, I'd ask him why his character concept doesn't fit in to a role such as rogue, since, as others point out, he could be a religious zealot. I'd also point out that his actions (standing at the back, throwing daggers) fits the striker role which is a different role than the one he has selected.
Assuming he's trying to "prove" that 4e is broken by preventing him from creating a particular concept, you just need to make him understand that his "proof" is only valid if he follows the instructions in the book. Otherwise, he's not playing 4e and he's just showing that his incorrect interpretation of the rules is broken

He'd be just as wrong if he did the same thing in 3.5. In fact, with total reliance on healing from a cleric/paladin in 3.5, he'd probably be worse off.
Personally, I find character classes to be a complete abstraction; something that the "converting your character" series on the D&D site seems to confirm. At the end of the day, the name and even the description of the class are fluff and the only relevant bit is what the class is supposed to do (it's role). As far as I'm concerned, the cleric could be non religious, the fighter could actually be a monk and the warlock can be a witch; using a little multiclassing where you need to blend two themes together.
I asked a recent new-to-4e player what he wanted his character to do, and he described how he wanted to run in to battle, and through the sheer grace and power of his attacks, rally his allies. I pointed him at the Warlord. Disgusted with the name and role of "leader", he dismissed it - saying it was just someone who ordered others around. I tried to convince him otherwise, and he just wasn't having it.
So I told him I'd write his own class for him and asked for suitable names for the class and role ("Knight" and "Inspirer"). I went away and came back 45 minutes later with the "Knight" (a level 1 warlord, with all the power names changed).
He loved it and has been happily playing since. Whenever he levels up, I transcribe any new warlord powers, give them flashier names and he picks out what he wants
That said, the game is up if he ever studies a PHB too closely. At some point I'll have to come clean
