I think you need to analyze his talking. Is he talking because he is a social kid, or is he talking because he has to have the last word? These are two VERY DIFFERENT types of talking...but both are pretty serious.
Children who have to have the last word, always with a response to everything an adult has to say to them are being incredibly disrespectful. They are behaving in a way that says, "My words are just as important, if not more important, than what you have to say." If your son cannot accept correction without feeling like he needs to explain why it isn't his fault, or has to justify everything that he has done, and he can't stop talking no matter what until his words are out, then this is a serious problem. It needs to be dealt with seriously because as your son gets older, it will only get worse. It will turn into arguing with adults and will cause him to lose all sorts privileges later. If this is the kind of talking he does, then you, as a parent, need to retrain him. You need to never allow the arguing or "explaining" in the manner that gives him an ego boost of having the last word. You need to work on his being respectful at home with the adults there.
If your son is just chatting to everyone about everything going on and is just being social, then you have a different problem that is not as serious as the one above, but still a real problem.
So analyze the problem here. What your son doesn't seem to realize (or care about is that his talking is causing a huge distraction. It is rude to the teacher. Even if it is the second, seemingly less important type of talking, he is still distracting other students who are trying to learn. He is being willful and disobedient. Is his purpose to make the others laugh? Does he think he is clever and funny?
You describe your kid as a good kid and you say you don't want to discipline him. That is obvious! Having him eat a vegetable is not a discipline for misbehaving at school. So I would re-evaluate this and treat it as a very serious issue. Learning self control is a VERY SERIOUS thing, especially for a young man.
Perhaps your son needs to do his work in the hall. Perhaps he needs his desk moved away from the other children until he learns the self control he needs to earn the privilege to sit with his friends again. Talk to the teacher and ask for suggestions. Ask her what kind of talking it is, and ask for examples of when it happens and how. If your son has the need to be funny and disruptive, then you need to work with him on how to fit into the group in a different way.
All-in-all I would not treat this matter lightly. I would talk with your son very deeply and earnestly about his talking...what kind of problems it causes (distracting those who are easily distractable, making teaching harder for his teacher, being disrespectful and willful.)
I would also put your son in a sport the minute he hits junior high with a coach that makes them run or have opportunities for unpleasant consequences for the kids who have failed citizenship. I know personally of several kids who behaved beautifully in class just to avoid the consequences in athletics.
Good luck...BE CONSISTENT!!...Treat it seriously.
VickiS