K.R.
here are some ideas
make sure he has appropriate places to beat things up :
* we have foam swords, with which the children--but really the boys only do it--are allowed to whack the couch, the walls, the doors ... but not anything that would break or is alive. They can whack each other's swords if they are fighting
* another thing boys seem to appreciate is digging ... sacrifice a chunk of your back yard (if you have one) to a digging pit (I have to reestablish this at my house)
* be sure to tell him "Hey, Josh, go beat up the couch" not "stop hitting Jimmy!" (I usually yell STOP!!! first to break the focus on the hitting, but then as soon as they are kind of listening tell them what TO do (instead of focussing back on what they shoudn't do ;) )
timeouts can work a different way than standard--he needs to stay in his time-out space until he is ready to be gentle with people and the dog ... in order for that to happen, he has to have things in his time-out space that calm him down and allow him to process and re-center (books? stuffed animal? Transformers? gentle familiar music? blankets and a pillow are a must because sometimes they are just tired ...). Sometimes he will need a parent to help him recenter (probably especially as he learns the new version of timeout!). And if he comes out and is not being gentle, he goes back in ... not a punishment, just a reality-check ... suggest all the calming things ...
OR, use the old-fashioned method for boys :) and send him out to "chop wood" ... what is a big-motor-skills activity that is (or appears) useful to your family that he can be set to to physically reorganize himself?
on the deeper end, my oldest son had sensory integration issues that led to an inability to read other people/understand when they were hurt, he didn't really know where his physical body was/ended so he always ran into things, he didn't process language intuitively, and his brain would lock down if I got too loud/intense (unfortunately I used to yell a lot when frustrated, and a kid who couldn't listen and couldn't be careful was very frustrating) ... thank goodness I figured it out (as well as making changes to get myself under a bit better control as well, still in process of course) ... he was a fairly extreme version of a set of neurological patterns that is NORMAL for boys (we diagnose it as a disorder now because it's inconvenient to our assumptions of what children 'should' be like, sigh) ... but he was (and is) actually the most gentle spirited of my children, and just as sweet as the rest ... the aggression really can be simply testosterone, even at this age, and at this age they have no tools to redirect or control themselves!
Alright, that's all my thoughts for now ;).