Clean the wound and then try cover it. Preferably with gauze but an old shirt or towel would work as well. This way flies don't get into the wound and infect it and so that he doesn't peck at his own wound. I've read that you can put cornstarch or baking flour on the wound if you do not have styptic powder. This will help with any bleeding. I'd suggest keeping him inside for a few days so he can recover. Once it starts to scab over, you can put him back outside. The wound should heal in a couple of weeks. Just check to make sure it's not infected and clean if necessary.