metagaming was the worst thing that could happen in role-playing

Well, this is just plain wrong; not having fun is the worst thing that can happen. It sounds like you're finding this out.

I think you recognise that there is no right way to role-play or, more precisely, there are as many wrong ways as there are gaming groups because no group is consistent in the way they play anyway.

Personally, I hate the term metagaming because no one agrees on just what it is and so people use it as a catch-all derogatory term for anything they don't like. In a role playing context, metagaiming is:

an "out of character" action where a player's character makes use of knowledge that the player is aware of but that the character is not meant to be aware of.

and more generally:

Metagaming is any strategy, action or method used in a game which transcends a prescribed ruleset, uses external factors to affect the game, or goes beyond the supposed limits or environment set by the game.

What you describe doesn't strike me as (definitively) metagaming. A player saying "Jim won't bear the thought of not getting xp from this kill, so he'll save me" is not metagaming, its joking around. There is a legitimate in-game reason for the action which goes like this: "Jim's character is my character's friend, so he'll save me"; which is independent of the out of game joke.

At worst, this is breaking character, not metagaming; I actually think that this is deliberately breaking the fourth wall, a legitimate dramatic or comedic technique (Deadpool: A fourth wall break inside a fourth wall break? That's like, sixteen walls!). The distinction is important because its possible that you agree that metagaming is bad but disagree that breaking character is. The first step in dispute resolution is working out what the actual dispute is!

The next step is talking it through. Explain to this player, as a group, what you like in your game, acknowledge that this is not the way that he plays and that this does not invalidate his way; it just invalidates it at this table. Try and be reasonable and have a sense of humour about it, perhaps you can give him little signs on sticks with "Metagaming", "Character break" and "Fourth wall" that he can hold up as a silent protest when you do it. Let his character earn XP for this when he gets it right, that will really warp his mind. By the way, this is definitively metagaming.

You have to take this step even if it means losing him to the group because the alternative seems to be losing the group as a whole.

metagaming was the worst thing that could happen in role-playing

Well, this is just plain wrong; not having fun is the worst thing that can happen. It sounds like you're finding this out.

I think you recognise that there is no right way to role-play or, more precisely, there are as many wrong ways as there are gaming groups because no group is consistent in the way they play anyway.

Personally, I hate the term metagaming because no one agrees on just what it is and so people use it as a catch-all derogatory term for anything they don't like. In a role playing context, metagaiming is:

an "out of character" action where a player's character makes use of knowledge that the player is aware of but that the character is not meant to be aware of.

and more generally:

Metagaming is any strategy, action or method used in a game which transcends a prescribed ruleset, uses external factors to affect the game, or goes beyond the supposed limits or environment set by the game.

What you describe doesn't strike me as (definitively) metagaming. A player saying "Jim won't bear the thought of not getting xp from this kill, so he'll save me" is not metagaming, its joking around. There is a legitimate in-game reason for the action which goes like this: "Jim's character is my character's friend, so he'll save me"; which is independent of the out of game joke.

At worst, this is breaking character, not metagaming; I actually think that this is deliberately breaking the fourth wall, a legitimate dramatic or comedic technique (Deadpool: A fourth wall break inside a fourth wall break? That's like, sixteen walls!). The distinction is important because its possible that you agree that metagaming is bad but disagree that breaking character is. The first step in dispute resolution is working out what the actual dispute is!

The next step is talking it through. Explain to this player, as a group or 1 on 1, what you like in your game, acknowledge that this is not the way that he plays and that this does not invalidate his way; it just invalidates it at this table. Try and be reasonable and have a sense of humour about it, perhaps you can give him little signs on sticks with "Metagaming", "Character break" and "Fourth wall" that he can hold up as a silent protest when you do it. Let his character earn XP for this when he gets it right, that will really warp his mind. By the way, this is definitively metagaming.

You have to take this step even if it means losing him to the group because the alternative seems to be losing the group as a whole.