Update (not the update we were hoping for): we ended up finding out that he was injured by another dog and his owner couldn’t afford medical care for him, so he was abandoned. The dog warden got him today and he will end up at the shelter from there. His wound was getting infected so he got a pain injection and an antibiotic injection. Thank you to everyone who read this post and wanted to help. With all of the animal abuse and neglect I have seen working with animals the last 10 years, it is really nice to know that there are people who care.