Short and simple, the reason he does this is because:
1. He gets a reaction...and sometimes the car swerves around like an amusment park ride! FUN!
2. It is his version of a tantrum. It gets him immediate attention, even when you think you're ignoring him...he knows that you are disconcerted by his screech. Little ones are quick to pick up on body language.
So what to do? Well, I hate to be redundant, but I suggest that you use the same technique I have recommended to several parents to stop tantrums (and it works!!!). Don't be afraid of the unconventional! This technique has been handed down a few generations in my family.
First: Get a few small plastic cups and put about 3-4oz (half way full) of water in each of them. Put them in the fridge to get nice and cold.
Second: When you adorable angel busts out with a scream go immediately to the fridge and get one of those nice cold cups of water. Don't let him see it! Hide it behind your back.
Third: Quickly walk up to him and dash the water in his face. I guarantee he will stop screaming in shock. HIDE THE CUP behind your back again!!! He should NEVER know where this cold-water shocker comes from!
Fourth: He might start crying. IT'S OKAY! Let him. Go and get a towel and clean him up, but NEVER explain what happened. Take your time getting the towel so that the full implication of what happened can sink in. Don't talk to him or coddle him too much. He should only associate his scream with the sudden shock caused by the water. This needs absolutely no explanation and will only confuse the situation if you try.
So, to break it down Barney-style: Cold water in the fridge. Kid screams. Get water without him seeing. Dash it in his face. Cup is NEVER seen. Comfort him and dry him off. No talking about the water or telling him "now don't scream anymore." Hugs and love. Repeat when necessary.
Rules: The idea here is to get him to understand that his screaming will have a VERY negative reaction. It might take only once for him to understand this, and it might take a few more times. It depends upon your child's temperment. Either way, it won't take long at all for him to put two-and-two together and realize that every time he belts out a scream he ends up with cold water in his face.
So the main rules are: NEVER threaten the water after he has screamed. Dont' say "If you scream again, I'm going to get the water!" It should come seemingly out of nowhere and he should never have cause to be afraid of you due to the fact that he won't understand where it came from.
And again, never let him see that cup.
After the water has been thrown in his face, never explain what happened. Act like it NEVER happened. Just get a towel and clean him up and tell him "uh oh! Okay, stop crying. You're okay!"
Finally, DON'T FEEL GUILTY! It freaks some moms out when they see the look on their little one's face after they get that water in their face (honestly I had to stifle a laugh!). Just remember....this WORKS! And it doesn't hurt him in any way.
Give it a try...and if you have any questions please contact me!