Hi there,
Here is an idea that I almost wound up using with one of my ninth grade students. I think it's more elementary (pardon the pun), so I thought I'd share it with you.
First off, see if you can touch base with his next year's teacher. And even see if you can get this started now with his current teacher. Give your son, let's say three, four or five tickets -- tickets to be used when he absolutely has to tell a story or share something. These tickets are NOT to be used when he as a question that is relevant to what he is learning. Rather, they are tickets for sharing a story about the concept he is learning or something else. The teacher then collects the tickets. He only has those many opportunities to share or be "chatty," if you will, each day. Make a chart for him that the teacher then can put three or four or whatever smiley faces on to confirm that he only used the amount of tickets he was originally given. Give him incentive when he gets home by providing immediate, daily rewards. Nothing huge to break your bank, but maybe a fun activity that he likes to do with you. Or maybe an extra story at night or a couple of extra minutes for a show. At the end of the week, reward him with something big -- since the weather is getting nicer, maybe take him out to the park for special play time after dinner and before bed. Or maybe take him out for a water ice or ice cream. You get the idea.
Also, you can get the teacher on board by brainstorming special privileges that he can get while in school. It might sound lame to us, but maybe one could be erasing the chalkboard. Kids -- even highschoolers! -- love stuff like that. Or writing the day or date on the board.
By doing all of this, you are working on modifying his behavior, which is ultimately what you want to do. You don't want him NOT to share his thoughts and ideas. Rather, you want him to start to think about when his sharing is appropriate and to evaluate whether what he wants to share is important enough to use one of his tickets.
And what if he comes home with extra tickets? Like if he doesn't use his tickets for the day? I wouldn't carry them over. You got me here. I'm sure you could think of something, though!
Much luck to you and your sweet son. You have the opportunity now to teach him how to use his gift of gab in a positive way -- a way that will keep him involved in his classes.
T. :)