I was going to bring up language which I see another person has. Has he much language? Often the frustration of not being able to tell you exactly what he wants is enough for extreme tantrums.
And unfortunately it's time that needs to pass until he is less frustrated.
I agree with others in trying to assess he needs with the help of professionals. Knowledge is power.

My son is just entering the tantrum phase and man can he scream!

It is hard and from the sounds of it you are being very patient!