Does it look infected at all? If not, you may just be able to rinse it with some warm water and leave it be. If it looks infected though. (Hot to touch or puss.) I would clean it first with warm water and maybe apply a bit of antibiotic ointment on it to start if you have some on hand. In the morning though, (if it looks infected) you may want to go get some antibiotics for him which are available at your local feed store. Oh! I also read somewhere on BYC that people recommend Apple Cider Vinegar added to their water as it has antibiotic properties or something. Fresh garlic too. Do a search on BYC with "Apple Cider Vinegar" or "garlic" and you may find something. I have never had a chicken with a open wound yet so I am not exactly sure.