I am so sorry that you all have to go through this - even him! Of course it is NOT normal behavior. Your son would NOT do this if he could do handle things another way. The problem is figuring out why this is the only way he seems to be able to handle things. I am NOT saying "poor little boy" and telling you to coddle him. I promise. I AM saying that there has to be something causing this because it is incredibly rare that a child, esp at age 6, would choose to act this way when he was capable of handling things in a more socially acceptable way.
Your family needs to write out a safety plan for when he rages. Who goes where, who does what. Little sister goes into a room behind a locked door (that SHE locks if possible - my youngest has to start doing this at about 3 when Wiz raged) and she has things she loves to play with and things that soothe her and movies that she can play. You and husband should each have a phone in your pocket or on your person at all times when you are home. Even in your jammies in case he rages then. Sit down and decide what things will result in a call to 911 (not for difficult child to go to juvie but for transport for an out of control mentally ill child to the hospital or psychiatric hospital) - I would say any acts that would hurt him or someone else or destroy property should result in this call.
You also need to document each event. Consider which areas of the home he most often goes to when he rages - often it is where the family spends the most time. It might also be where MOM spends the most time. Often a difficult child will rage more for/at Mom than anyone else. It feels like they hate us and we are their target. Reality is VERY different. They rage at/around us most because they know we love them no matter what and will love them even when they rage. We are the safest thing in their world and they bottle up or save whatever is going one and then let it all spew out with us. Not any fun, but it does make sense. I often thought this and in the last year my difficult child has opened up about things he did/said/thought/felt back in the bad times. Wiz said this was exactly why he would be "fine" at school, with my parents, at friends' homes, in stores, etc... and then explode at home - esp around me. (He has been doing a lot of thinking and looking back at those times off an on since he turned himself around.)
If you have an area where difficult child rages more - family room, kitchen, your room, or wherever - consider putting a video security system in place to record that room. I just saw one at Sam's for $149.99.. You could mount the cameras unobtrusively when he was at school or one of you took him out somehwere, and then have a record of it that he wouldn't know about. It would show what he was doing and neither you nor husband would have to be holding the camera. We found that when Wiz was raging he would run from the camera or attack it. He was rarely able to just stop, but many kids can. they just stop when someone official shows up or they see a camera, so it can be very hard to get video.
The books recommended are excellent. I also recommend strongly that you read "What Your Explosive Child is Trying to Tell You" by Dr. Doug Riley. It will help you figure out why he is exploding and this can help you figure out how to stop them. He really has a handle on what we go through and the kinds of things we need and need to know to help our kids.
As you are setting up evaluations, be sure to include evaluation for sensory issues by a private Occupational Therapist. Some people's brains do not handle input from their senses the way most people do and it can create enormous problems. My kids all have them to some degree and I do too. Sensory Integration Disorder (SID) or sensory processing disorder (SPD) (sensory integration or processing disorder - seems to be called one or the other depending on the source you are looking at but they seem to be the same thing from what I can figure out) can be treated fairly effectively and the treatment is often pleasant and/or fun for the child, so there are not as many battles over it - from what I have been told. For me, when the sensory issues are really bad it feels like the world is attacking me - sounds, lights, smells, even the texture or pressure of my clothing all feels bad or somehow wrong and is super hard to cope with. It is even harder when you are a kid and don't know why all these things are such a problem when everyone else is fine with them.
I hope that you can get the help you need. It may take repeated calls to 911 to have your son transferred to a psychiatric hospital or other hospital, but this WILL help you get the docs to pay attention. Consider also looking for a developmental pediatrician. They have special training in what should be happening as a child develops, identifying problems in development and treating these problems. Even if your child hit every developmental milestone, he could still have a problem taht the dev pediatrician could help with.