My DD1 who is now 10 suffers the same affliction. I think I possibly tuned her out a little too much when she was younger as she now talks without checking first that anyone is listening (not good socially). So we really work on making eye contact and stressing taking turns and listening as well as verbal-outpour.

So my advice is -
* Set up some quiet zones (ie when the tv news is on, or when I'm sitting in this particular armchair means I just need some quiet time)
* Teach your son about pauses in conversation, getting someone's attention, making eye contact and then talking when you're sure they're listening. Practise this at the dinner table and at other times when you're doing things that you don't mind listening to chat at the same time.
* Teach about talking about one topic at a time then it's the next person's turn to talk (kids this age just go from topic to topic to topic and it's hard to tell if they've finished saying the first thing LOL).
* realise that your son probably has an auditory way of processing information. Basically you're getting word for word what's happening in his head. Give him other auditory things to play with (in his bedroom hehe) - musical instruments, audio books, a tape recorder so he can record things then play them back, his own radio/stereo/mp3 player, games with noise-components to them. These will all grab him like nothing else in the house does and at least you'll have some quiet when he's off playing with them.