Are there any good herbal remedies to help calm down my very aggressive autistic son?

No. For patients with aggressive symptoms as part of their autism there are no herbal medications that have been shown to be of any value. Certain drugs like Risperdal or Clonidine have been shown to be of value.
No. Are there any good herbal remedies for calming autistic kids? The real answer is no. None of the stuff in other answers has any proof of effectiveness we think they are safe, but no safety studies have been done. We would like simple answers for a complex problem, but they often do not exist. In 35 years of taking care of 100s of these kids, I have yet to see consistant effect of these therapies.
Yes. Fish oil and probiotics are great for children and adults with autism. They are safe and effective. Valerian and lemon balm are relaxing. You can also try craniosacral therapy and reiki.
Not proven. Nothing that has been scientifically proven to a great extent in kids. I dont disagree with the recommendations from dr. Garber at all, but even these dont have great, solid scientific studies to show they really work on a grand level. These are very safe however, so if they help your child in safe doses, then try them.