First of all, well done to all the people who designed this champion, I think this is now my favourite champion (and I've only played him a few times) His gameplay design is awesome, and comboing E with transforming and ulting is an awesome combo (although I'm pretty sure this was a bug you decided to keep :P). His boulder feels awesome to throw and pick, and all of his abilities have really awesome mechanics.
Now even though he's awesome, there are some things I would change:
* I really like that you can throw the boomerang and hit someone behind you, so maybe you could make that mechanic worth using and make it a bit easier to pull off:
* Make the boomerang return to where it was thrown instead of how it is now. This would make it easier to land.
* Make the boomerang have more range on its way back so that it's worth the increased cooldown. Something like this:
* I've never liked abilities that reduce their damage each time they hit something. I feel bad for hitting minions, and hitting someone alone doesn't give enough good feedback to fix that problem. I'd prefer a boomerang with reduced damage that deals the same damage to everyone than making it hard to guess the damage it will do to each target. If it has too much waveclear, you could reduce the damage dealt to minions.
* I don't think the attack speed buff on his E is necessary. It feels strange and that power could be allocated somewhere else.