"They started throwing stones at him."

What would be the passive form of this sentence ?
The throwing of stones at him was started by them.

The meaning is possibly ambiguous here. It's not clear whether they started throwing stones at him or just gave the signal for the others to begin throwing stones. At least that's how I see it now.
How about "He started being thrown stones at" ?

Is this correct ?
Site Hint: Check out our list of pronunciation videos.
He started getting stones thrown at him.