My three-year old daughter does the same thing. We reasoned. I tried teaching breathing techniques. I hugged and cuddled and held until my back gave out. I was ready to tear out my hair!
Finally, I read a technique in "Making the terrible twos terrific" -- we tried it, then made our own adaptations, and it seems to be working.
First, when our daughter starts screaming, we take her to her room. We say "you can stay here until you stop screaming. When you're under control, you can come out." (The threat of closing the door was enough to keep her in, but you may need to close the door for your son.)
Not having her screams blasting in my ear from the next room really solved 90 percent of the problem for me!
It took a while and we nearly gave up -- going to her room seemed to make her scream more, but we reasoned that she's a smart kid, and she's making the choice, so she must need it in some way.
Just last week, she started telling me she needed to cry, and asking to go to her room. Usually, she's completely under control, and even talks cheerfully as I unlock the baby gate, then she goes up and fake-cries for a few minutes, then plays happily for about a half hour before she asks me to come up and get her. She's thrown a couple authentic tantrums.
When she comes down, she's cheerful and very proud of herself, talking about how she calmed herself down.
At first, she did this three or four times a day -- as if she wanted to test that she really could calm herself down. Now, the timing is getting predictable -- after she does her chores in the morning and when I ask her to eat something she doesn't like at dinner time. I think she really just needs time to regroup in the morning, but is trying to assert control at dinnertime. So in the morning, I let her to to the room. At dinner, I've started telling her she can go up, but she has to stay there until bedtime. She's chosen to stay and eat, reluctantly, but without tears.
Whatever path you choose, I would advise totally eliminating drama, sidestepping power struggles, presenting it as his choice, not as a punishment, and giving your son room to solve his own problems rather than trying to solve them for him. I am much happier and calmer now than I was 6 months ago -- and so is my daughter.
Now if we could just solve the potty problem and the eating problem, life would be bliss!!!! (or the kids would come up with something else to bedevil us with :-)