My husband does that more and more, finishes my sentences before I get to, he inserts things I was NOT going to say.
I feel for you, cause I know how agitating/exasperating this is. I have told my husband repeatedly not to finish my sentences for me, and do not assume your inserted words is what I was going to say. We have gotten in arguments over his rudeness. We've been married for 28yrs and know each for 35yrs, going back to our teens, perhaps he is too comfortable.
I think we need to remind these guys, we can speak for ourselves, and will when we need to or want to.
My husband was never like this till he hit his 40's.
Nipping this with your son now, the sooner the better, may help prevent him from being obnoxious and rude in his future. I wish I had a solution, but I don't, I can relate to what you are saying, and empathize.
Take care and lots of luck with this,
DE