The first step to resolving any such linguistic quandary as this is to look at the etymology of the word(s) in question, in this case “damsel.”
The word damsel comes from the old french word damisele which comes from the latin word domina, which translates literally to mistress, but is meant by the “female form of master” defintion.
The male form of the word domina is dominus, which translates literally as “master.” Since the morphology from latin to old french only changed the ending of the word, it’s safe to assume that the masculine counterpart would have followed the same morphology.
The old french ending of “ele” is equal to the modern french ending “elle,” and the male counterpart to “elle” is “el.” Typically the two spellings are pronounced the same and are only written differently. When heard spoken rather than read, sentence structure generally informs the gender of a word of such words.
Logically we can infer that the male form of damisele would have been damisel, spelled differently, but pronounced the same, and by extension it’s Middle English derivative, and therefore English derivative, would undoubtedly have been damsel.
In conclusion, “damsel” is the male equivalent of “damsel,” and by extension, “damsel in distress” is the male equivalent of “damsel in distress.”