I think Pielorinho got it right. Sure, if the guys dragging the game down with his constant preaching or somesuch, get him to lay off. But if he digs the paladins, what's wrong with that? Let him go crazy. As a DM, you can use it as a base to flesh out your world. Make his god require that he joins an order. Give him some superiors in that order. Have them give the paladin tasks, some cool, some boring, some that the rest of the party will balk at. Mix it up and add structure to the class. The player will either love it (good for you) or decide to stop playing paladins (good for you, I guess). I'm DMing 2 paladins right now, and I can assure you they are mostly broke all the time and have numerous tasks before them set by their order that they might not normally do, but in a character context works just great. They've built shrines, taught local classes about martial training, donated tons of money to orphans, and they love it. Give it some thought.