Mouthy 9 year old- suggestions??

So i have a rather mouthy and argumentative 9 year old that i dont know what to do with anymore. Its not like the kids you see on "Supernanny" but his mouth gets him in trouble more often than it should and im at a loss on how to curb it. Example of the arguing: C- What time is it? Me: Its 6:30 C: No its not, its 6:28 Me: Close enough C: Well im just saying your wrong, its 6:28, not 6:30 And this is just one small example. if you say his nose is bleeding, he would say "NO, its not," even if it was. He ALWAYS has to get the last word in on everything. Its really annoying not to mention that it isnt really a great quality to carry with you through life. He does get disrespectful and everytime he does i explain that isnt the way we talk at our house, its unacceptable, do not do it again. Well, im sick of talking and telling him over and over and over. at age 9 he knows what okay and what not. I shouldnt have to give warnings and explain time and time again how what he said or the way he said it isnt acceptable. I have in the past put him in time outs (which he's too old for timeout now at 9) sent him to his room when he cant be respectful, say, Try again, until he says it without whining or a snotty tone of voice. And they all really work for the moment, but nothing has curbed the behavior long term. Im usually a very no-nonsense parent and he doesnt get away with anything but i havent found a good solution to this problem. I really, really want to get him to understand that it just isnt right to speak to people the way he does and that later in life it certainly isnt going to do him any favors with a mouth like that. Especally when he argues with my parents or dh's parents, that really gets me mad cuz he has no sense of respect for his elders. He just thinks its okay to say anything to anyone. And that isnt how i was raised and it really urks me that he thinks nothing of the way he talks to them is wrong. Even though i constantly tell him not to say this or that. I need action, but i dont know what action to take!!! Does anyone who has kids this age have any advice for me?? It isnt going to get better the older he gets, i want to get it under control NOW!! Thanks,Rach