A.,
I would highly suggest that your husband take your son out for a father son weekend. I suggest a weekend (with just the two of them not the other boys) because if your husband and your son are away your husband will not be distracted by mowing the lawn, phone calls, work, you and your other kids and your 8 year old will have his dad all to himself for a whole weekend.
I think your son will think that is very special. My dad took me on a couple of father daughter trips, just the two of us and they are some of my very best memories I have from my childhood.
When you take a child out of their element to some place special with just one parent that child has 100% of their attention. When a child knows that you are a safe person and will listen to what they have to say they will talk and open up more than they would with their sibling around.
I would suggest that they spend the first day together, but not address the anger issues and then spend the second day together and later in the day after they have spent a good amount of time together have your husband just ask him why he is hitting and kicking and so angry all the time. And then just listen. He might have to ask more leading questions as 8 year old don't self analyze like adults can and do. Maybe it's something happening at school, some tv programs he's watching where violence is being modeled, maybe it's your relationship with your husband (I have not idea).
I don't know what your son enjoys doing but pick something that he will like to do and think about the memories that are being formed as well, despite the fact that your purpose is to probe deeper into this problem.
I am not sure but your son may be old enough to once the problem is discussed to be able to talk about what consequences would be appropriate for his actions if he slips up and does some of the same things after the weekend. If he is part of the deal then he is more likely to stick to it.
I would also suggest that you make this a regular yearly or twice a year thing, to just spend time one on one with your kids. It really leaves lasting memories that are so special I wouldn't give them up for anything!
B.