Was he provoked? Was it in play, and he just got too rambunctious? Perhaps he just didn't like the sound of the whistle, but I honestly think something else must have happened. He probably doesn't even realize anymore why he is sleeping in the garage. I'd never do that to my dogs. They'd be corrected and put in a 'time out' for a short while (simply to relax, and calm down), and that would be the end of it. I don't see why he should be put to sleep, just going by your description..