Maybe try giving him something that it's ok to throw and a safe place to do it in? Encourage him to get his fix and master the skill with soft foam balls in the backyard or something?

In addition to continuing on with the discouraging of throwing inappropriate objects - that way it becomes a substitution thing, and you can redirect his penchant for throwing in ways where others aren't in the firing line and aren't as likely to get hurt.

He'll grow out of it eventually, unless he's the next Brett Lee.