I think Mark gave some excellent advice. Maybe your son talks a lot because he just wants to talk about everything under the sun that he has seen heard, etc for the day. I wouldn't think to discourage him either, doing that could have him retract into his own shell and that could be worse. Just my two cents though.
Teaching self control via marital arts is a great idea as long as you know your son won't use it to harm others.
I agree with Mark. Also, I would recommend you try to have him take any martial arts lessons. They teach focus, discipline, self-control amount other great things. He'll probably have a hard time at first being quite during the class, but if you help him with consistency, taking him to his katate classes it'll do wonders for him. Also, it's great for self-esteem.
Both of my daughters have been doing Taekwondo for three years. Best decision we ever made.
That is really good advice from marklakewood.
First off, I don't necessarily see a child talking a lot as a bad thing. I would be more concerned with a child who keeps to his/her self. So, I'm not sure that it is a good idea to discourage this behavior.
On the other hand, if your son talks inappropriately, interrupts others, talks back, etc..., he would be breaking rules for which he should be disciplined. Discipline helps kids learn self-control.
Socially, I would explain to him how excessive talking may hinder relationships. I would also role play with him how to talk and have conversations with others in order to make/keep friends.