Can you take away whatever he throws? We went through a throwing stage (although not at another sibling - he's still cooking!) and each time ds threw a toy I'd take it away. We put it up on the fridge, where he could see it, be reminded of how much he wanted it back and how he'd better not do it again. Most things went up there for a set period of time for the first 2 times then it was up there until the next day.

Good Luck!