I don't have any personal experience on the situation with your boy. But I would like to add that, if we turns out that he gets diagnosed with mental disorder (ADD/ADHD) or something else, you should know that there many ingredients in our diets that are associated with aggressive behavior in children.
It is up to you what route to take (alternative medicine using diet/supplements vs. traditional medicine using drugs,) but jumping into a psychologist or psychiatrist right away would most likely end up into the path of taking medication, and he is too young for that. Maybe you should see a Naturopathic doctor.
Do some research online on behavioral problems such as ADD, ADHD, autism in children and diet. You would be amazed what the food industry is feeding our children! Its not just the sugar...