If his behavior is not characterized by anger or aggression, and he is mostly bothered by the impact of saying No to the impulse to talk, he may display ADHD. Now, it's important not to jump on the ADHD bandwagon, so to speak, but to undertake a thorough evaluation to determine if he meets the criteria for the diagnosis. One of those criteria is the sort of incessant talking you are describing. When ADHD is a legitimate diagnosis, and environmental/behavioral measures in and of themselves have not been sufficient, stimulant medication can be quite useful. At his age, if the diagnosis is confirmed and medication is recommended, I'd suggest trying a longer acting rather than a short-acting stimulant medication. Methylphenidate, the active ingredient in Ritalin, is also the active ingredient in some of the longer-acting preparations. Possibilities would include Concerta, Ritalin LA, Metadate CD, for example.
Perhaps my experience will help you, perhaps not. I have two children who talk constantly. One of them is entering middle school this year.
Both children were recently evaluated by the school for learning disabilities. Because of this both children have been seen by many different professionals in the school system. There were lots of different thoughts about their different learning differences, but none of these addressed the issue of talking too much.
THEN the speech pathologist saw one of them. She immediately recognized the signs of "Nonverbal Learning Disorder". The big red flag here is talking all the time, and not necessarily about what they are supposed to be talking about - this disorder evidently is caused by deficiencies in the right brain function. This disorder is also recognized by a great difference in IQ scores, for instance both my boys have very high verbal IQ scores, and significantly lower (20 points less) performance IQs.
I am no expert, am just learning about this disorder, but essentially I understand it in this way:
The child attempts to compensate for their right brain dysfunction by increased activity in the left brain. They verbalize everything, and try to distract or "make-up" what they cannot do. It is common for these children to have difficulty with organization and they are often called "lazy". They usually have a great store of information in their heads, and can discourse about many different topics. Many of them do well in lower grades, but in higher grades when they are asked to derive their own conclusions, and environments are less structured, they begin to fail at their tasks.
These children are generally good at learning things by rote (memorization), have trouble reading social cues, and often are told "I shouldn't have to tell you this." There are lots of other elements to this disorder, which is a true, neurological disorder. It is not something that goes away, and these children need guidance for their whole lives, learning the acceptable ways to behave in differnt situations, and active participation in their school environments.
I went ahead and purchased a book about this disorder from the LinguiSystems, Inc., at 1-800-577-4555, called The Source for Nonverbal Learning Disorders. The book cost me $50, but it was worth it, because was sure at least one of my children had this disorder. I don't know if it would help you...they have a website listed. www.linguisystems.com. The book is written in a very approachable style, with lots of definitions, and talks about how to work with your child in school, and about parenting these very bright and challenging children.
My child had already progressed in this disorder to the anxiety stage...seeing things "black and white" is another one of the symptoms...and when you are constantly criticized for something that you thought was right, it causes the child to begin to worry about not doing things perfectly all the time. Depression is also common. So we are already in a psychiatric practice for help...the practitioner says this book is just "more stuff like Mel Levine", which i understand is at www.allkindsofminds.com but I have not purchased any of his books...I am very happy with the one i previously mentioned, and the suggestions it holds appear to be helping me and my children.
sorry if this does not help, but it was worth a shot! Good luck!