I would try Pro-biotics, there are strengths for children though I have used full strength on my daughter and my dogs and never had any bad side effects. Doctor's won't prescribe them for the most part, but if you ask them about it they will almost always say yes that is a good idea. Go to a health food store and get the good ones like Jarrow or Udo's.
So, we went to the childhood psychologist and were not shocked to hear ADHD with some Anxiety. Mostly because his dad also has very classic symptoms of ADD and I have some of the more aggressive ADD behaviors. (We are a fun household, truly) It was almost a comical light-bulb moment listening to my husband with the family questionnaire saying "but that is normal kid stuff" and me replying "maybe for you but to me telling an 8 year old Antonio, 'do you have your shirt on yet?' 'Where are your socks?' 'Why don't you have on pants yet?' 'Antonio did you brush your teeth?' etc. all morning long instead of 'get ready for school' everyday is a bit much"
It also goes a long way towards explaining why those two (Dad and son) bicker the way they do. We will see how this goes, I know it is a common diagnosis, but it fits so well. For now we are treating the ADHD and getting some parenting help (when & when not to "help" him) in hopes that this will resolve the anxiety to a level that he can manage on his own.
I have been forced to become more organized as a parent (I never was pre-kids). School mornings are all about routine: breakfast, brushing teeth, getting dressed, pick up room, study upcoming quizes (spelling, math,etc), and if the kids have done everything they watch TV, off to school.
We treat TV as a reinforcer for work. Work first and well then get TV.
Does he like animals? Most kids around here love em'.. try the DVD of Milo and Otis. Kind of borderline on the animals follies but, my kid has learned to talk from it and calls out all the names; etc..
check my last comment too on the group main-page
I'll second the idea of seeing a psychiatrist. Also, play therapy can be really helpful, but, like anyone working with your children, you'll want to vet them some first. Another thing that might help a little on the fringes is coming up with a coping strategy list with him. After an outburst, when he's completely calm, talk to him about how he feels when he loses his temper. Validate his feelings - especially that he might feel scared when he loses control. Then talk to him about what things help him calm down. For example, when he goes to be alone maybe he's holding a stuffed animal or staring at a picture in a book or even just zoning out staring at the wall. Help him to make a list of the things that calm him down. The next step is to be ready to help him get to his calming strategies before he escalates. It'll take careful management for a while, but you may find that if you can help him have a few successes at calming down before he's completely out of control that he'll have more confidence that he can control his anger. This may actually reduce how often he has the tantrums. For some kids, the tantrums are exacerbated by fear that they're going to lose control in another tantrum. By giving him some successes at heading the tantrums off, you may lessen that fear and, thus, reduce the behavior. Eventually, after a few months or so, you may even be able to help him go through his calming/coping strategies in his imagination. For example, after he's spent a while avoiding or deescalating from tantrums by holding his teddy bear or hiding under a blanket, he may be able to take some deep breaths and imagine holding the bear or hiding under a blanket and get the same feeling of calm and safety.
Good luck! Remember, patience, love, support, and time almost always pays off in the end.