This is how my sister got her cat last year. He was brought into a shelter with severe wounds on his neck (much worse than in your photo) and it turned out that when he's stressed, he self harms by scratching around his neck area. I'd go with the idea of putting a soft collar with a letter attached asking the owner to get in contact. If the collar is still there after a few days, you'll know he's a stray and can hopefully get him treatment.

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Owned by Ratty girls Hera, Athena, Boomer and Kitty the Cat
RIP Apollo and Starbuck, together again at the Rainbow Bridge. Till we meet again.