I recommend the book "Scream Free Parenting." He has a website as well:
http://www.screamfree.com/
I also recommend a schedule posted on the fridge for when your son is home. That will eliminate those pesky questions and you can refer him to the schedule. Here's a sample:
3pm - come home from school and show mom school mail
3:15pm - have a snack
3:30pm - do homework
4pm - 4:30pm - computer time
4:30 - 5pm - clean up before dinner
5pm - dinner
6pm - 7pm - family time
7pm - 7:30 - pack backpack for morning, bath & get ready for bed
8pm - 8:30 - maybe tv time
8:30pm - bed time
You get the picture. Kids like to know what to expect. It's comforting to them. If the routine is going to change, let him know in advance. Or have different routines for different days. Change as needed, but not too often.
When he is calm, I would sit him down (make sure your husband is there as well) and tell him the new rules. You could post them (with pictures) as well.
Rule #1 - No hitting - EVER!
Rule #2 - We treat everyone with respect and courtesy - NO EXCEPTIONS!
Explain that now he knows the rules, there is a zero tolerance policy and he will be sent to his room for XX amount of time. Explain that he has a right to disagree with you, but he has to do so respectfully. I think that is really important. Ultimately, you and your husband make the decisions, but your children can have a voice too. They need to learn how to voice that opinion in a respectful way.
Now, the door swings both ways, A.. You and your husband need to treat your son (and daughter) with respect and courtesy too and most of all - no hitting. "I understand you're upset, honey, but you cannot talk to me that way. Go to your room now." You also need to be consistent and follow through. I know it's hard and we all get worn down, but you have to toughen up or else he'll be worse when he gets older. Nip this behavior in the bud now.
I would also take everything out of his room except his clothes and his bed. He will have to earn all his toys back by being respectful and not throwing tantrums. Develop a reward system. No hitting and 5 days without a tantrum will get him 5 toys.
The other thing I might do is enroll him in TaeKwonDo or Karate if he's not currently involved in any activity. Both disciplines involve teaching respect for persons in authority and treating yourself with respect. Did wonders for my oldest girl. She loves it and has a much better attitiude (she's 10).
Good Luck and let us know how it goes.