I'm sure there's a Greek pronunciation AND an anglicized one. I'm guessing on the English version, en-DIH-mee-uhn (?). And the Greek, en-di-MEE-uhn (also, ?).
Hey, for interest, in French it's pronounced Aw(ng)-DEE-MEE-aw(ng). I like it! The "nickname" would be, Endime, pronounced Aw(ng)-deem, or, Dimi pronounced, dee-mee which is like a French version of "Jimmy". An alternative spelling using French pronunciation is, Endymione, which would make the last "n" say it's sound -- pronounced Aw(ng)-DEE-MEE-ohn.
I think it would be a great name for a boy because it's so unique, no one would have it! Although, there aren't any good nicknames that go with it, but I personally don't use nicknames much.
Endymion was Mamoru Chiba's name in his past life on the Japanese manga/anime Sailor Moon. Prince Endymion of Earth fell in love with Princess Serenity of the moon, but their love was forbidden due to the fact that they came from different worlds. Princess Serenity is said to be a descendant of Selene, paralleling the Greek myth.
https://en.wiktionary.org/wiki/%E1%BC%98%CE%BD%CE%B4%CF%85%CE%BC%CE%AF%CF%89%CE%BD