I know some adults like that. It is VERY irritating and after awhile you don't even want to talk to them or be their friend.
Its really intrusive... you can't even finish a thought... before the person barges in on what you are saying or thinking.
In adults... it is seen as Arrogance and self-centered obnoxious behavior.
In children, up to a certain age, it is excused as a maturity or developmental reason, and it can still be "cute." But after a certain age... habits like this is just seen as, very irritating and soon others will not want to talk to you. And it is not excusable anymore.
Anyway, well CAN he just learn (by now), to RAISE HIS HAND when he wants to speak or has the answer or wants a turn???
My son is 6 and is VERY talkative & expressive, but he will raise his hand, even at home. And he is fine at school, never any problem and well behaved and never talks out of turn. His Teacher tells me this. He waits his turn or will ask for a turn.
Some people, interrupt others because they... want to have the one up on the other person, or they can't wait and it is an impulsiveness problem, or they only are thinking about themselves & it is selfish, or they always want to be the one with the answer FIRST etc.
But for your son, it is a continual... problem.
What other impulsiveness issues does he have???
Or speak to the Pediatrician.
He is in 3rd grade already.
Is it only NOW that he has this problem or has he always been this way?
I would also try... in telling him that WHEN he wants to say something... TO WAIT 5 SECONDS before he barges in on another person's conversation... and then ASK if he can have a turn.
I am sure, he knows by now... that it is very rude. But he still does it or disregards the other person.
If my 6 year old can do it, your son can too.
Why don't you just ASK your son WHY HE DOES that???
Instead of guessing.