the last word

Nathan's need to always have the last word no matter what is astonishing to me. He's been told time and time again that sometimes he just needs to STOP TALKING. Yet, he continues on. He got sent to his room 3 times last night for this. He always needs a reason, he always needs to put in his two cents and he always needs to go "that extra mile" to get his point across. I wish I understood the need for this. It is one of the most frustrating things I deal with. As I understand it, this is just going to get worse as he gets into puberty. I've never been a "because I said so" kind of parent even though I think that's a good enough reason sometimes. His need to keep harping on a subject or explain his side is mind blowing. For example; we had friends over last night and the kids were all playing video games. Nathan is very competetive and will run his mouth constantly as long as he's winning. He does this to the point that he's annoying and I fear if he does this at school someone is going to slug him. Anyway, if he is losing it's another story. He's whiny and everyone else is cheating and it's not fair. He was told last night to "stop running his mouth"....he kept on. I sent him to his room and he did NOT stop talking the entire time trying to explain his side. I walked out, closed the door and the door to the hallway and could still hear him talking. It's very frustrating to say the least.
I worry that he will never be able to hold a job when he's an adult because he will continue this behaviour. We've taken things away, grounded him, gone to therapy....he just NEEDS to have the last word. I'm at a loss at this point and can only hope that continuing to point out that he need not have the last word always, will eventually sink in. My biggest fear for him is that he will unknowingly sabatoge himself with his quirks as he gets older. He's so smart and has so much knowledge to share and a true gift to share,if he could just learn to reel it in alittle. I want him to be happy,productive and successful so we will continue,through trial and error to find a healthy balance of what is Nathan and what is socially acceptable.