I was going to recommend a touch of Neosporin, but Pevidine or Betadine would work well if there is a wound. If it's just skin, hold his tail in water for 30 seconds or so, and it should peel right off. You might have to use tweezers to grab it, though. I'm sure the snake would be fine either way, but wild snakes very often lose a bit of their tail. I think it's a good idea to do what you can for it.
P.S. I recognize that little garter by the suspicious lump in its belly
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