Maybe he's just trying to get your attention; let him know that tantrums aren't the way to get it. And if he continues, ignore him when he has tantrums. Once he sees his tantrums aren't getting him the attention he's aiming for, he'll probably stop.
Excellant advise from lisamoon and , get him doing a lot of physical exercise ,games and running , basket ball, have some guy times with him,try to focus on the positive side of his behavior.and ignore the Tantrums.
I have a 9 year old boy who has been doing things of the same nature as your son. It is almost like day and night with my son. Nice one minute and the next not so much. Not to dicourage you in anyway because my son is 9 and still having problems. I recommend talking to your childs doctor, or even a Child Psychologist. When nothing else was working for us that is what I had to do. I tried sports, and still ignore the tantrums. I even hand out cards to people who stare at me because I'm not stopping his behavior the way they think I should be. It just tells them that some children have issues that cannot be controlled and it has nothing to do with my parenting and they should not pass judgement on things they do not understand. I have 3 other children who do not have the issues my son has, so if you have tried everything in your power as a parent you should take the next step and talk to his doctor. I do hope that it is only attention that he is demanding and everything works out for you and your family.