According to all timeline information, the First Era was inaugurated by his reign. He must have been awfully important in his time to mark the beginning of an era. Was he a Bosmer or an Ayleid king? And more importantly, could he have been a Dragonborn Ruler like Miraak and St. Alessia?

Interesting question, as far as I know there aren't any texts or evidence pointing to what race he was, I believe he was welcoming towards Ayleids into his empire though, but that proves nothing as he was regarded as a benevolent king anyway. Personally I believe he was a Bosmer, as it seems most fitting that Valenwood was united by a Bosmer, not an Ayleid. I think one can conclude since there is no evidence that he was an Ayleid, he was probably a Bosmer. The same can be said for the Dragonborn notion, there are no tales nor any evidence that Eplear was a Dragonborn, and usually history picks up on who is a Dragonborn and who isn't.

I'm curious as to what sparked your thought of him possibly being Dragonborn, though?