I KNOW WHAT YOU ARE GOING THROUGH!
When my daughter (now 17) was about 3, she was like this. I have 3 children (17, 14, and 6) and have seen temper tantrums. This was not a normal temper tantrum or even a temper tantrum. The best thing I could find to compare it to was what some people call "night terrors". (It did not happen at night, usually in the middle of the day.)
She had this problem for months; raging fits where she was completely unreachable. I tried everything I knew to do and many of the things already mentioned by some of the other moms. She was a great child before and after and no one would believe me when I asked for help. When I tried to describe what she was like, they looked at me like I must be exaggerating. She was never a behavior problem for anyone, but very loving and talkative. After a while I quit telling anyone about it; I was ashamed, thinking it must be something I had done and I was a bad mother. I even hesitated to post this message, not wanting to relive the judgemental attitudes I faced.
I am a Christian and I prayed for her through all of it. At one point I even questioned to myself "is she possessed by a demon"? The reason my weary mind even jumped to that thought was this: It would start from out of the blue, sometimes when she woke from a nap and sometimes for literally no reason at all. When she was throwing these "rages" her eyes were completely bloodshot. The episodes would stop as mysteriously and as suddenly as they started and immediately her eyes would be clear. (very freaky) By the way, she was completely unaware of what she had done when it was all over. She did not remember anything.
Her episodes would never be shorter than 30 minutes and could last as long as an hour and a half. She would be completely content one moment (or asleep) and in a complete rage the next. I am not sure she could even hear me when I tried to communicate. She did not want to be touched and she did not want to be left alone. She was not hitting at me or trying to hurt herself, she was only screaming at the top of her lungs.
All I can tell you is, she finally stopped having them after about 6 months. I do not know what started them or what ended them, only glad they left.
I have a friend whose son went through this for a much longer time (years). As he got older and could communicate, things got a little better, but he became violent to his brothers. He ended up seeing many doctors, including a neurologist. He is doing better now.
All this to say: Hang in there, hopefully it will pass or he will grow out of it. Keep asking your child's doctor about it until you get the help you need. Good luck.