Hi, I am also the CL on the ADHD forum here -
http://www.medhelp.org/forums/ADD---ADHD/show/175 - feel free to ask any related types of questions over there. For right now, I think that your Pediatrician is on the right track. However, I have seen kids on the other forum who work so hard to hold it together at school that they just melt down when they get home. So I think I would talk to his teachers and get a better feeling for how he is doing at school. Getting A's and B's really means very little. I have seen lots of intelligent kids with AD/HD do that in first grade. About 4th grade things began to fall apart. So talk with his teacher about his attention span, time on task, frustration, etc. And, I must say, you mentioned a few things going on at home that set off warning bells for me. The main one being, "Additionally, when something happens, he says it is not his fault and tends to blame me a lot or anything near him." Kids with either ADHD or ADD can do things impulsively or without thinking. Many times they may not even realize what they have done - until its too late. After a while they get tired of being blamed and start making excuses, lying, etc.
What can you do at home? There is an excellent set of books which are aimed at this age group. They are meant to be read aloud to the kids and then practiced. You might want to look at "Hands are not for Hitting" , or "Talk and Work it Out", or "Cool Down and Work Through Anger". You can find information on those books here - http://www.amazon.com/Hands-Hitting-Ages-Best-Behavior/dp/1575420775
Basically, you will be trying to show him how to change his behavior and express his anger. It won't happen overnight. But, if you get both boys involved, it will make a difference.
Hope this helps. Please post if you have any more questions - either here or on the other site I monitor. Best wishes.