Welcome. I am glad you found us!
Given the late milestones he achieved, and the fact that you think he may be bipolar, I would like to STRONGLY recommend you find a neuropsychologist to thoroughly evaluate your son, or have him evaluated at a Children's Hospital by a multi disciplinary team (group of docs who evaluate). He will have many hours of tests, often as many as 10-15 hours broken up over several days. This will give you the BEST idea of what is going on. And teh best chance to help your son.
I recommend reading The Explosive Child and suggesting that school use these methods also. I also recommend The Bipolar Child - if nothing else it will give you a starting point to try medications with (and if medications are being given by a pediatrician go and get an appointment with a board certified child and adolescent psychiatrist - these medications are WAY out of the comfort range for a pediatrician to be prescribing).
MOST of us, I would say over 95% of us here, first had our kids diagnosis'd with ADHD and then we found it was much more. Bipolar is something you want to pay attention to early on, because medications for many other things, including adhd, depression and even colds can cause someone with bipolar to cycle. PLEASE don't try the antidepressants (ad's) until you have tried teh mood stabilizers WITHOUT the ad's. Ad's can make a child TOTALLY unstable and it can take MONTHS on mood stabilizers to get them stable. There is even a type of bipolar that is caused by the SSRI/SNRI medications like prozac, strattera, zoloft etc... So trying the mood stabilizers first could be VERY important!
You should probably also do some research on autistic spectrum disorders. Autism is NOT the stereotyped image of a kid who completely doesn't interact with the world. It is an entire spectrum of behaviors. There are a LOT of things that can help a child with a form of autism - my oldest has a form called Asperger's Syndrome and he is currently a senior in high school, graduating in May. He will spend next year at the technical college completing a machinist training course and then will work as a machinist to pay for college! (And from grades 3-9 I had NO HOPE that he would escape life behind bars, college wasn't even in my wildest dreams for him at that point!!).
You do NEED to go to school and make a BIG STINK so that the school KNOWS that you will NOT tolerate them comparing him to ANYONE! It truly isn't fair to him. He probably feels there is no point in trying because he CANNOT control a lot of this and the school always points out how different he is, how much "less" he is. It is no small thing, what they are doing to him when they compare him to other kids. My oldest was bored and very gifted. So he caused trouble and talked. The teachers compared him to other kids and did it right in front of him. The pressure got to be so bad that he made several serious suicide attempts in 2nd grade - AGE 7!!! If nothing else, remind school tehy have a burden of confidentiality and they are NOT to compare his work or behavior to other students as it violates his rights. I am so sorry that this still happens.
Homeschooling can be a very positive option. I homeschooled my oldest for grades 3 adn 4. In addition to saving his life, it gave us time to track down therapy and medications and go to all the doctor visits with-o hassles from the attendance office and we became very close.
One thing you DO have to realize is that your son is NOT like everyone else. His brain is probably wired differently. It means he will have to learn in different ways teh stuff many kids pick up by osmosis - the social stuff. Often it takes a LOT of practice to learn this stuff. You may find that social skills groups can help - ask at a children's hospital about this. There are a lot of good books you can work on at home to do this also. Others can give you titles, it has been a LONG time since I had need of them and I have forgotten.
Anyway, welcome, you will find we are a pretty supportive and fun group. Glad you could join us!