How to handle a very defiant 8 yr old?

I am not positive because I don't know more than what you have put out, but it could be a slim chance that he has a traumatic, or hurt feelings that he is (or has) pushed down very deep. He could be acting out to mask feelings he does not want to deal with, or is ashamed of. If there is someone that he trust to open up to. Find out what is making him tick.