Try a 'no-crow' collar. They're on the market, or you can make your own. I took 1" Velcro, cut the length of the rooster's neck, just under his head, plus two finger lengths, under his feathers. Then wrapped it around his neck and secured it to itself. It's a humane way of stopping him from taking a full breath before he crows and he cannot get the full effect. Make sure you get his feathers out from under the Velcro, as this will make it ineffectual, it has to be up against his skin.
He'll sound funny for a while, and if on long enough, he'll learn that it's useless to crow anymore and not do it as frequently.