Throwing a hard object at the back of somebody's head as a 9-year-old is not excusable. I would suggest some kind of mild punishment - either extra chores or a loss of some kind of privilege - that lasts long enough to leave an impression on his mind.

But, as others have said, emphasize that you love him and his sister and you want them both to be safe and healthy. And you want him to learn an important lesson and ultimately be a better person, and that's the reason for the punishment. Nothing about anger or revenge or anything like that.