So, one of my PCs is a Lawful Neutral Knight, coming from a small landless noble family.

From the beginning of the campaign he told me that he wanted a lord to serve under, and to enhance the prestige of his family.

However, later on, he directly told me OOC that he was more or less a greedy bastard who cared only for power.

What he tells you and what he does are two different things. His internal motivations, either as a player or as a character, are his own business and shouldn't concern you as a GM. What should concern you are his actions (player or character).

It's a nice dream to be able to plan subtleties into your campaign which are geared toward the internal motivations of the character, but that player needs to know that if he changes his mind as a player or is deceitful in any way it will not only frustrate your efforts but you as a person.

He let you believe he wants one thing, then went in another direction. You wasted your time working out ways to bring joy with his original ideas, and now have to ad-lib responses to his new direction. Sometimes players surprize a GM, and it's wonderful. Sometimes players are frustrating.

He does need to know that he has frustrated any efforts you may have made which took his original motivations into account. He as a person needs to grow.

That's fine as a character, but he swore oaths to numerous NPCs that he never intended to keep, declared his loyalty to the kingdom and its people, and in general deceived more or less everyone he met to his true intentions, all without rolling Bluff checks (I don't think he has a bluff bonus, if he does it's something like +3, so a passive bluff, if that's a thing, might not dupe an ogre). Is this a legitimate thing for him to do?

(A "passive bluff" is a thing. It's an untrained roll.)

Yes, it's perfectly legitimate. Sortof.

First, I personally wouldn't relate lying and bluffing too closely, but if the mechanics are clear on this then yes he would have needed to make those rolls, but only if you as a GM decided it was important to do so. Lying an oath to a major NPC should definitely require effort.

Second, the player is essentially retconning his previous vision (as he told you), which is a no-no. Doing it gradually is cool. Having an about-face moment in-game which is motivated by some occurrence in-character is damned good roleplaying. Just changing his mind for no discernible reason is not.

If the rules say he should have been rolling bluffs, and he retconned that he was bluffing, then you could retconn the bluff rolls in private. See if all those people he was lying to knew all along and were themselves bluffing their acceptance of his lie. Adjust their reactions to him and plans involving him accordingly. Completely mess with that player's head and plans. Where players take no small pleasure in accidentally derailing a campaign, a GM should take pleasure in being unsurprized and able to react unflinchingly to anything the players throw at them.

That player (and every player) needs to learn that they can't go back in time and say "what I actually meant was". They're at a fixed point in the game's reality and they are not allowed to go back and change anything.

It's ok for them to have a legitimate reason to change their character's/player's mind about something, so long as there is an in-game reason for it. Ask that player what his reasons for changing his mind are. Maybe there are some subtleties that the player has only recently worked out (which is awesome!) and you can help integrate his new vision into the game.