The payoff was funny but I really don't think it was worth a year of abrasive annoyance. If the secret paladin and the DM had just been up front from the beginning and made it obvious that the others were getting healed, feeling inspired, etc so that the other players could figure out what was going on sooner it still would have been funny and everybody else wouldn't have spent a year of gaming pissed off.

This is where restrictive definitions of metagaming detract from the fun of the game. Insisting that "your characters don't know he's really a paladin so the DM shouldn't even bother telling you what the effects he's buffing/healing you with are" is just BS. Maybe draw out the irritation for most of one session for the sake of a punchline but after that simply knowing that the guy is roleplaying as an abrasive ass rather than actually being an abrasive ass would allow most players to actually enjoy the experience and play along rather than getting constantly ticked off.

There's also the matter of the PCs, completely in character, not noticing that they're being healed or being emotionally bolstered against fear. Maybe once or twice in the heat of battle, but eventually there should be cases of "I just know I took a gash in my side there, I can see blood on my clothing, but the wound is closed." Presumably characters who live in a world where paladins exist would at least know what paladins are and some of what they can do even if they don't have previous direct personal experience with them. To insist that players should pretend their characters are ignorant is like telling them they aren't allowed to ask the person in robes and a pointy hat with an ornate staff and an extremely emotive and capable cat following them around if they can cast fireball because they aren't a wizard, haven't met any wizards, and clearly have never heard stories of people matching this person's description making things explode by pointing at them.

Metagaming is, by it's simplest definition, any behavior that involves acknowledging that you are playing a game. Pretending that fundamental mechanics (like healing or giving advantage against fear saves) don't exists is just an unnecessary hassle that overcomplicates things and encourages confusion. When you have to stop and think about how to describe something without using game terms, like "I feel as if two more hits like that last one could incapacitate me" rather than just saying "Crap, I'm down to 15 hp," that breaks immersion for the sake of...not breaking immersion. The PCs should have had plenty of chances to realize what the paladin was doing, and figuring that out much earlier would have made the game a lot more fun for everybody for a long time.