The question is a bit unclear and I’m not quite ready to delete my other answer since I’m not sure it’s inappropriate, but I think this would get lost in it and may be closer to what Zach wants, based on comments.
Therefore, I am answering two questions here:
How should I tell a player that he’s not playing a character the way he should be played?
How should I deal with a player who describes his character as one thing, but plays him as something else?
I separate these two because they are different question, and need different answers.
1. How should I tell a player that he’s not playing a character the way he should be played?
Simple: you don’t. That’s none of your business. If a Paladin needs to Fall because of his actions, then that’s what happens (but see my other answer for thoughts on how best to handle that mechanically – I despise the official rules on the matter, and never recommend them), but it’s because he’s not being a Paladin, not because he’s not playing how you imagine the character. It’s his character, not yours.
In effect, when you ask how to do this without seeming to accuse him of bad roleplaying and/or trying to play his character, the answer is you can’t, because that is exactly what you would be doing. Maybe he is roleplaying badly; maybe that accusation isn’t unfounded, and maybe that’s a discussion you actually should have. But if he’s not roleplaying “badly,” there shouldn’t be any need to comment at all.
2. How should I deal with a player who describes his character as one thing, but plays him as something else?
This is a different matter. Particularly when the character’s backstory touches on the rest of the world, as your player’s does with his father as the head of an order of paladins, there is a greater cause for concern and a greater need to set things straight. Remember, while his character is his, the setting is yours. If your setting does not include any order of paladins that would condone or accept his behavior, then he can’t be a member of one – or he is about to get into trouble with them.
And if he was raised to be this way his entire life, and up until now upheld those standards, maybe they’re going to worry if he’s been cursed or possessed or something. But maybe he hasn’t been – maybe he felt stifled in the order, and now that he’s out in the real world he’s feeling a bit restless and rebellious. Maybe he’s looking for a bit of independence from a father in whose shadow he’d spent his entire life, whatever.
But the key thing is for the player to recognize that his actions do not jive with his backstory. I suggest that you give him these choices:
Change how you behave, to be more in line with how the character was described in the backstory.
Change your backstory, to make your character a bit more rebellious or a bit more light-hearted; maybe his father hoped that going out into the real world would make him a bit more serious.
Keep both the behavior and the backstory, and accept that this is very much an abrupt change in his behavior. Tell him that the order will not be amused, and will be greatly concerned about it, possibly even angry.
All of these things, however, have more to do with the order’s rules, and not the Paladin’s Code of Conduct. In both the second and third case, however, warn him that Falling is a very real concern for the order. They do not appreciate his behavior precisely because they believe it will cause him to Fall – and point out that putting others in danger needlessly is something that could cause him to Fall quickly.
But like I said in my other answer: a Paladin’s allowed to have a little, or even a lot, of harmless fun, so long as it is harmless, and he remains that unwavering bastion of Good and honor. Throwing a chair at someone or throwing an ally into a group of enemies may not be harmless, but there certainly are plenty of “un-Paladin-y” things he could do (e.g. harmless pranks) that will never cause a Fall.
A Radically Different Idea
Your player had fun playing a greedy awesomedwarf? And now isn't having fun playing a Stop Right There! Paladin? Easy.
Have him take that in-character.
Whoa, whoa, what are you talking about?
Simple. Have the paladin start to be dissatisfied with the course he's taking. Have him start bucking authority, becoming a dangerous loose cannon. Going a bit far. Being reckless. I'm directly quoting Lethal Weapon here, because that's what you want to be. Martin Riggs is the ideal Paladin-walking-a-darker-path. He wants to be Good, and Lawful, but there's just so much darn Evil out there, and it's hurting the innocent. So many good reasons to just cut that moustache-twirling evil guy's head right off, and not give a damn.
Maybe the Captain of the Town Guard calls him in and lectures him and threatens to just stuff him in prison - maybe it's his direct superior in the paladin order. Either way, you do your part by having authority figures drive him further towards vigilantism, violence, and using evil means to defeat evil, and he can do his part by playing a conflicted, confused young man treading a path that leads to darker acts than his foes can even dream of - all in the name of Good.
And then you end up with something a bit deeper than a black suit of armour with a different flavour of boring character inside.
Best Answer
Address the problem at the source: Retcon1 the story. If your players demand an in-story explanation, remember the origins of the owlbear: "A wizard did it."
At the end of the day, all the participants involved are aware that the game that they are playing is a story. The cleanest solution, therefore, to an external (non-narrative) story influence that is just messing things up is to fix it outside of the story.
Simply say "Hey, this encounter? Here's how it really happened. This way, $Player can play $character and everyone can have more fun." It's the honest way, it hurts the least, and it allows everyone to just get on with things.
As a matter of protocol, I give all new characters a "retcon cookie" (but only one) that can be used at any time to adjust their character sheet in any rules-valid way. There are elements of a character that only come out during play that can just be seriously annoying for everyone involved. There is a tacit understanding that this cookie should be used to resolve a player's issue with her character, not as a "oh, we need to get around this in-story problem... I was a rogue all around." But it's quite acceptable if that restriction is spelled out.
1 A definition of Retcon:
For your purposes, the retcon would leave the events of the story the same, but simply indicate that $Player's new character did them instead of the old character, thereby preserving continuity.