I attend Positive Discipline meetings with our Mothers of Multiples club and I have received some great tips and tricks that worked for my children at the age your son is at.
We taught our children sign language to enable them to communicate with us sooner than verbal skills would allow them to. The improved communication definitely lessened the frustration on their part. (Sign language for simple words - which are all they know right now, are available on DVD with either Signing Time or Baby Signs).
When our children would throw a tantrum, I would say, "Oh, you're mad! You're mad, mad, mad!!" As I'm saying "mad", I would shake my head to emphasize the feeling. This would usually result in them cracking up...but, it gave them a word for what they were experiencing. I would also grab a piece of paper and a pen or crayon and tell them to show me how mad they were. I would put the pen or crayon to the paper and scribble while I said "mad, mad, mad!" It is completely disarming for them. They start to scribble, get the frustration out, and then move on.
As for the hitting, we did what Deann suggested. We would grab their hand (typically in mid-hit) and say "No hitting" firmly. We took it one step further and then while holding their hand, would gently stroke the person who was going to be hit and say, "We touch so-and-so gently." This doesn't work overnight...but, it definitely works.
Another solution is redirection. When he becomes frustrated or angry, simply take his hand and say, "Oh, look! What's this?!" Simple diversion works well in many cases, too.
I hope that some of this helps. Hang in there...as it is only a phase, and does get better in time!