Good idea. Maybe this would have been better in colour, as I feel his head and hand don't jump out of the photo enough. There is a lot of grey here, so that the skin tone grey merges with the background a little too much.
Other than that, a well composed image for the challenge
Voted earlier, just coming back to comment. I immediately hear/think the word stop when I look at this. However, I wish either all of the guy or at least his hand was in focus. The hand, to me, is what is saying the word the stop. Personally, I feel the whistle is more of a sound/noise than actual language. 6