The big thing IMO is to be very, very firm, consistent, and CALM. If you get upset, you'll be teaching him that that's an appropriate response to the situation.
That being said, I totally believe in (and needed, with my strong-willed ds!) zero-tolerance rules. NO hitting, etc (whatever limits are right for you.) Tell him no clearly, state very simply what he SHOULD do, and immediately remove him to a time-out or similarly calm (and pre-introduced) consequence. Like, last summer when ds would break the rule and walk into the street, we immediately stopped outside play and went inside. No matter what. That worked quite quickly, let me tell you! For less serious misbehavior, we counted to 3 and then did time outs. But you HAVE to be prepard to follow through, or you can make it all worse!
Last idea is to try to praise positive behavior every chance you can get. Not like random, "oh, you're just generally awsome in every way" stuff, but like, "wow, great spoon holding!" "gee, you are SO good at being gentle with your sister!" It'll reinforce good behavior, and help you get some positive interaction in there once in a while!
It's hard, and 3 was even worse for us (sorry!
) Good luck!