One of my hens were attacked by a dog, had some pretty nasty puncture wounds on her back. I kept her isolated, gave her baby vitamins (polyvisol without iron) just put a drop on the beak and let it roll off for it to drink. keep him warm and pack it with some sort of antibiotic cream. if it has been a while since treatment I wouldn't stitch it for risk of closing infection inside. I just packed my hens wounds with a cream I bought from the feed store and let it close up on its own. It might take a while if it is big/deep. Is it in an area that he will peck at it?