While the kid seems immature, he is, well, a kid. So that's to be expected. What I can't figure is why everyone is holding the OP blameless in the situation.
I mean his character concept is that he gullible, a lunatic or a gullible lunatic. Dressed in aurthurian myth or not the core of his character concept is that he suffers from a delusion. He doesn't bother to explain this to any of the other players, apparently because he prides himself on his roleplaying. When he reveals his delusion, the DM asks for a know nature check. (not know history, sense motive, know nobility or anything that might allow him to give the other player some hint of what is going on.) His character defends his delusion, becoming more extremely delusional, rather than give the kid a break and step out of character for a moment to explain what is going on in the conversation. No, "the fine art of staying in character" requires that he continue to act in a completely irrational manner.
Wow, what a shock that the kid got frustrated and just went for PVP. :\
I wouldn't have either the kid
or the OP at my table based on this story.