I agree with doing a Neuropsychologist evaluation. His anxiety level sounds beyond age-appropriate. The odd things he did in Kindergarten would attest to that as well.
Forms of autism CAN look like ADHD too. Kids who are high-functioning are often accelerated in academics, if not gifted - and this often hides the disorder.
Severe anxiety is a hallmark condition of ASDs too. And, ADHD medications often don't work in kids that have ASDs.
I am going to get that book that has been touted here - "Helping Your Anxious Child". It looks and sounds good. I've had 4 debilitating panic attacks over the past 15 years - and was able to stop one last week. My son's anxiety isn't as profound as your little guy's (mine's the same age) - but I know he's going to need some techniques to cope.
Good luck.
