D.,
Oh, I know how the screaming can just crawl under your skin and drive you nuts! My second child, a girl, just turned one and recently discovered her screaming voice. She has always been loud. Man! What I do know is that my little one is not loud so as to get extra attention- she gets tons of attention. She does it because she likes it. It sounds like your son does it as a way to express himself, no matter what the emotion.
I have a friend who dealt with screaming in the car like this: she stopped the car, told the child that the car would not go until the screaming/shouting stopped. They were stationary for almost 2 hours, but it NEVER happened again.
My Love and Logic book suggests something similar for car scenarios, changing it somewhat by stopping the car and getting out- you the adult (the child stays in, safely strapped in the carseat). Act like you're having the time of your life reading a book, smelling flowers, whatever. But send the message that when the yells start, you get out and then he misses out on something fun. This is also done with a very pleasant cue, like, "oh dear" or "uh oh" just prior to your action. It's without any lecture, very little explanation, and no angry venting.
Could that type of strategy be transfered to your situation at home? Maybe you could get some headphones, and when he starts screaming, smile at him and say, "I love you and I want to hear what you say. I also love my ears too much to let you hurt them. I'll take these off when they stop hurting!" Then completely ignore him while they're on, continuing with your activities like you're having a terrific time, all the while ignoring all requests, pleas, etc.
And of course, praise him when he's using a pleasant voice!
Good luck to you!
J.