First consider that your child might have ADHD. Read about it here (search ADD and ADHD or click this adhd tag) If it's likely, have the child tested and take things from there as well.
Mind you, I'm pretty proud of his loquacity.
It sounds as though loquacity may (consciously or unconsciously) have been encouraged by you. This is no different than delighting in baby's other achievements, but we don't delight in a 6 yr old's ability to walk anymore. It's time to teach and reinforce what words are actually for: communication. Also teach what they are not for: a means to get and keep someone's attention.
If you do not, less polite people might (by their actions) let your child know they are disliked for their rudeness (as is being done at present by his peers.) Please take this in the spirit it's given: I care for your child's self-esteem, and ideally that self esteem should be based on something of value.
First, I advocate strongly for giving a child a rich emotional vocabulary, so that they can identify the great variety of feelings they have, which is the first step in addressing them.
I would take a five pronged approach:
- Stop admiring the ability to speak for long periods of time.
- Practice concise speech and model it for your child. Make sure to avoid unnecessary interruptions.
- Identification of problematic verbosity (discussing why and feelings).
- Gamification/praise for concise speech and good communication; and finally
- Teach the importance of manners (to show others the respect one would want others to show oneself), which requires self-control (also praiseworthy.)
I love words, and one of my favorite words is laconic: "the skill of saying much in few words." According to Plutarch, Philip II of Macedon called on the Spartans to surrender, sending them the following message: "If I bring my army into your land, I will destroy your farms, slay your people, and raze your city." The Spartans sent a messenger with the answer: If. Phillip did not attack.
Example of approach #3:
Child: interrupts you and goes off on a long tangent....
You: "(loving nickname), I heard what you said, but I have a question for you: why did you say all that just now?
Child: "What do you mean?/Huh?/Because I wanted to!/other"
You: "But what made you want to say it so badly that you needed to interrupt me?
Keep at it until you get an emotion underlying is interruption, but you need to teach these emotion-words first. Some possibilities: Because I wasn't interested in/bored with what you were saying, because I was excited about x, because I was feeling impatient, because I wanted you to listen to me, because I wanted to feel important (I feel important when people listen to me..."), then discuss the behavior-emotion tie as non-judgmentally as possible. Repeat every time this behavior occurs. He will either find the reason he does this, give up, or change the subject (which brings you back to the first step.)
Example of approach #4:
Child: goes off on a long tangent....
You: "(loving nickname), I heard what you said and I'm interested, but can you say it again in fewer words? If you can say it in (reasonable number of words), you get a sticker (with nice enough rewards for x number of stickers so as to truly encourage him.)"
You: "Let's see, does that mean (you say it in many fewer words)?"
Child: "Well, yeah, but-"
You: "See if you can say it in fewer words like I did, and see if you can get a sticker."
Example of approach #5:
Child: interrupts you and starts to go off on a tangent
You: "(loving nickname), did you just interrupt me?"
Child: "No, I just-"
You: "(loving nickname), I know you want to change the subject to x, but really, did you interrupt me?"
Child: "Maybe, but-"
You: "Name, wen I get interrupted, I feel lonely (or whatever is true.)
Child: "Lonely? Why do you feel lonely?"
You: "Because when no one listens to what I'm saying, I feel like I'm all alone, even though I'm not, and I feel lonely." (Whatever is true and not too promoting of the child's guilt.)
Do you think this is a lot of work? Because it is. But you want to help your child.
This is what I would do.
Also, read about how to help a child pay attention (and other sites) and how to help a child stop interrupting.
Also, I am not laconic. I wish I were.