Take a look at his diet - many kids (mine included) react to artificial colors, flavor, preservatives, etc. You can find healthy replacements for most of the "junk" type foods kids like (i.e., fruit snacks, etc.), keeping in mind that almost all processed foods have this stuff in it (e.g., yellow dyes in Kraft Mac 'n cheese). Also, he could well be allergic or intolerant to wheat (gluten) or dairy - those are the top allergies. I will note that yelling, screaming and hitting are really separate issues from having a child that is headstrong or determined. If the latter was the only problem, I'd recommend the book "Raising your Spirited Child" - you might need that later, but I'd put my money on him having reactions to foods. What you can do to figure this out on your own (and save alot of $s) is to keep a food diary and behavior during a meal (some reactions are fairly quick) and after a meal (30 minutes, a few hours, and up to 3 days for some reactions). If you first remove all the artifical stuff, then you can try experimenting to look for a reaction - for example, my son bought a red gatorade at school one afternoon - he (my strong willed son) decided he would drink it with dinner. In a 15 minute period, he went from being calm and sweet to being a monster - violent, etc.
Also, many, many kids are low in B vitamins, iron and magnesium - there are excellent supplements (again no artificial colors or dyes, preservatives, etc.).
If you can't figure it out on your own, I recommend Dr. M. Ann Block: www.blockcenter.com - they can get to the root of it.