might not be what a lot of people like..but take away the thing/s he likes most, for 24hours. I lock a specific number of my son's toys in my bedroom (or outside in the shed) when he engaged in aggressive behavior or broke something on purpose (aggressively or not). It is matter of fact consequence, no discussion not fuss. He gets upset I walk away if another behavior occurs more get locked and/or I add more time before he gets them back. Another thing to do is a a kind of over correction meaning if he hits some one he has to do a specific number of chores for that person. If he throws something or makes a mess he need to clean the entire room. of course use a positive reward system (i am a fan of using real money even if "buying" something is abartary cost) and use more positives than negatives (general thought of best ratio is 4 positives to every 1 negative given for a general idea).
Also knowing when he most likely to hit helps then teach him a better alternative. You don't have to hit you can take a break if you need, you don't have to hit you can tell them to move, you don't have to hit you can ask do that later, etc.

Thank you PandS and DadRob! - you both provided some very helpful ideas. The topic of consequences is a tough one, and I've felt a bit lost on what to do - even our therapists have not been too helpful with ideas. I figure they are probably a bit hesitant to offer advice on this one since every kid is different.....