You could be descrbing my daughter when she was younger! She was so incredibly volatile that I really thought there might be something wrong with her. She used to have tantrums that lasted for hours. Her diet was terrible, but good luck trying to get her to eat anything else! I even dragged her to the pediatrician for blood tests, thinking it might be hypoglycemia or something. She would be so sweet and funny and then turn into this crazy person right before my eyes -- I used to wonder, can you diagnose bipolar disorder in a three year old? I swear, some days I thought I was losing my mind!
The good news: She is now 11, and is WONDERFUL. She still fights with her younger siblings more than I'd like, but she's a straight-A student, mature beyond her years, with a good attitude about life. As a matter of fact, she just spent her own birthday money ($50!) to buy food to donate to the animal shelter, and she's about to get a big award at fifth-grade graduation for being such a good student and good citizen. She has been on Zoloft for about a year and a half, but I think most of the changes have come developmentally, with maturity. Sometimes you have to just wait until their emotional maturity catches up with their IQ -- smart kids get really frustrated when they have all these thoughts swirling around in their heads without the ability to express them in an appropriate way.
The bad news: It took a LONG time, and a lot of hard work.
We did counseling, which helped a little. At the very least, it reassured us that she wasn't mentally ill, and that we weren't doing something horribly wrong with her. The counselor was able to rule out things like Asperger's Syndrome, etc., so we didn't have to worry about that, and she gave us some strategies we could use to deal with her behavior. I bought every parenting book on the planet, including Love and Logic, and used those techniques (you may also want to read The Explosive Child). I was very consistent (I also had two younger ones, so, like you, as consistent as I could be). But the reality is, although it was important to do all these things, I had to do them for a very long time before I saw the payoff. The reason I am telling you all this is just to give you encouragement to hang in there and keep being consistent and keep plugging away, even when it seems like it is having no effect. And don't let it make you doubt yourself as a mom. If I hadn't had two other kids who weren't acting this way, I would have thought I was a terrible mother who was doing everything all wrong. Just do the best you can, love him, and try not to take his behavior personally.
I hope that helps at least a little!
L.