My daughter is very similar to your son.  She is 8 years old and was diagnosed with ADHD and has ODD tendencies.  She has been in and out of therapy for years.  I'm beginning to think she may have early onset bi-polar (rapid cycling).  She has phases of "normal" behavior with some contention which is generally managable followed by other phases of major disfunction.  During her disfunctional cycles she rages, biting, hitting, kicking, throwing things, screaming rude and unkind things.  She damages/breaks things and has been known to pull doors off their hinges and put holes in walls.  She is set off by the smallest things and I feel like when she is in one of these cycles we aren't able to live our lives normally.  Sometimes it seems like Dr. Jekyl and Mr. Hyde like behavior shifts.  However she only behaves completely out of control with me and my husband (her step-father), and partially out of control with other people she feels very safe with.  I think she is afraid of abandonment and does it to push our limits.
She was abused by her biological father at an early age and only has supervised visitation, but he rarely sees her.  This causes much of her anger and she blames my husband and myself for her father's lack of interest.  
I just get so emotionally drained by her behavior that I feel I need some help with additional resources.  Especially regarding the hitting, kicking, and biting.  I'm working to get her an appointment with a psychologist covered by our health insurance and I've discussed these issues with her psychiatrist, but he tells me it is just her personality, and that some people are unable to feel empathy.  That is hard for me to believe.  This is not behavior we can just accept and live with.  Time out is not stopping her and I'm worried she will eventually take it too far and really hurt one of us or her little sister.

Your description indicates that your son has many positive attributes, along with some neurointegrative conditions (ADHD, learning diability), and perhaps a problem with his mood. This should be evaluated. Many (though by no means most) children who display ADHD also display a Mood Disorder (e.g., depression, bipolar d/o), or a type of Disruptive Behavior Disorder called Oppositional Defiant Disorder. If he is not now in therapy, it would be prudent to arrange this. You could achieve clarity of diagnosis, and both he and you could receive help. He could learn about how to manage relationships and amend his behavior, while you could receive some guidance about how to manage the behavior.