I've been there and it sounds just like my son.  I knew something was wrong when he never grew out the the "terrible 2's".

Have him evaluated by a child psychologist.  Ours was diagnosed ADHD and ODD (oppositional defiant disorder) in Kindergarten after it got so bad at school.  Later on they added Educatonal autism and required a few minor accommodations.  Extremely smart and somewhat socially awkward.  He takes meds everyday.  We could not have made it without them.  DO NOT let people tell you he needs to be raised without meds.  In many cases, they don't have a clue.  You may have to try different doctors until you find one you can work with.

The challenge you will have is raising other children with the oldest having issues.  It does not provide a very good example.  Meds are a continual trial and error process to find right ones and dosages.  Dosages need to be changed with increase in weight.  It will be a lifelong challenge, but will be rewarding as well.  My son is now a senior in high school and near the top of his class and looking forward to college.  Good Luck!