Here's a different perspective from those who've already posted.

Modern american kindergarten is not a normal environment for a young child. We evolved over most of history in hunter-gatherer societies. In these groups children learn while roaming in multi aged play groups. This kind of environment allows for tremendous behavioral latitude and a wide range of developmental skills.

The part of the brain that allows a person to sit, focus, refrain from actions etc. is the frontal lobe. We know from functional MRI's that the human frontal lobe is still developing well into a person's twenties. How and when this development occures is highly individual but we know it's often later blooming in boys.

The requirements of American kindergarten are really based on what adults need. We need our kids to go to school so we can go to work. We need them to be there long enough so that we have less after school care hours to manage. And we are anxious about our children's futures so we feel the need to hurry them along in there learning.

Most Nordic countries do kindergarten in a very different way that we do in America. There focus is on the emotional and language development of the children. Academics don't come in until age 7. The days are mostly free play with guidance from a teacher. Even better, many offer "outdoor kindergarten" where children spend the entire day outside regardless of weather. They are guided and nurtured by a teacher. How do you think your son would do in such an environment?

All of which is to say it may not be your son who has a problem, it may be the environment you're asking him to be in. Certainly, even if he does have ADHD, or SPD, or mild autisim, he's giving you clear signs that the school environment is currently not a good one for him.

Consider pulling him from school for a year. Give him the gift of time to just be young and growing. Try a small in home daycare where he can have more freedom and a smaller number of other kids to socialize with.

Good luck and hang in there!