I want to echo the sentiments of TeacherParent and Michellea, as my own son has ADHD. While I'm not a doctor, I do think your son may have more than just ADHD to contend with, and it is important to see a specialist, preferably a child psychiatrist, to help make a proper diagnosis and to develop a treatment approach.

As Michellea mentioned, please do join us in the Learning and Attention Difficulties Group at community.greatschools.net/groups/11554

I agree with Teacherparent that you need to find a doctor that can help you find solutions for your son's troubles. I would suggest looking for a pediatric psychiatrist (not psychologist) or neurologist that has experience with ADHD and other similar issues. They can lead you through a treatment plan that may include some counseling, behavior interventions and strategies and possibly medication to help your child.

In addition to dealing with the medical world, it sounds like your child needs some specialized instruction in school in the areas of social skills, self regulation and possibly communication/self advocacy. These can all be addressed via the special ed process. What's more, once you are within the special ed system, your child is afforded legal protections when it comes to discipline and other issues you are facing.

Write a letter to the principal requesting a full educational, psychological, speech and language and functional Behavior assessment. This will give you data on your child's strengths and challenges. The school will provide you with an evaluation report that includes the test results, interpretation of the data and recommendations. You will meet with an IEP team to determine if your child is eligible for special ed services and an IEP.

As a team, you will write an IEP (individualized Educational Program) that includes a description of your child and his needs, goals to improve his areas of need and services and accommodations to help him improve and cope in school. You as the parent are an equal member of this team.

At some point you may feel that a residential placement is best for your child. But, at this point he is quite young and there are a lot of things that can be implemented in school and at home that may help.

PLEASE find a medical professional that will help guide you with some good options. PLEASE exercise your rights with the public school and request an evaluation.

In the meantime, join us in the Learning and Attention Group: community.greatschools.net/groups/11554 for more information and support.

CHADD is a tremendous resource for parents of kids with ADHD. Check out their website: www.chadd.org/

And check out this group for information about your legal rights and special ed process: community.greatschools.net/groups/77570

Your son is young to go to a boarding school - I"m not sure there are many boarding schools that would enroll such a young child. You say he was tested for ADHD - what were the results of that testing?
And a different doctor might be more helpful one who says you're the problem and washes his hands of the matter.
That your son hit other children for the first time is upsetting but that he did this right after starting a new school puts his hitting in a somewhat different light. Transitions are hard on all of us and switching kindergartens is a transition. Though he's young, even young children have a good inner sense of what's going on - I'm sure he doesn't like being the way he is and he likely knows why he had to leave his other kindergarten. Some part of him likely thinks he'll have to leave this one too and hitting is a way of keeping his distance - he doesn't want to get attached and he's sad and angry both.
If he was diagnosed with ADHD, there are medications that can help with that although givng a young child medications is a very personal and sometimes difficult decision for families. Trying child psychology might not work as your son's behavior isn't the usual behavior for children - a psychologist might have better luck trying psychology on him.
I raised one ADHD son - it wasn't easy and I hope you post back.