Have your son evaluated at an agency for early intervertion for children, sometimes because children cant comunicate, they go into trantrums, I know my grandaughter had similar behavior and much more, we took her to get evaluated at an early intervertion center, Look up PDD in children.
Not easy is it this ole parenting thing, but worth it,I am wondering if he gets plenty of positive attention, and how he gets on with his sister, does he go to preschool, are there any issues there with his behavior,also get his speech checked out there may be reason there for speech therapy,ask your Doctor what he thinks.