I think they never revealed it because they didn't have any good reveals. People who aren't into comics watching the show wouldn't get Jason Todd, and there weren't many other characters we knew that it would be fun to have him be revealed as.
if they didn't reveal maybe it was no one. they just had thought of no one for the job and made a whole new character, the whole mistery thing was only for thw show, not really meant to be a great mistery, otherwise it would have been answered or more obvious to answer
I'll jump on the bandwagon and call him Jason Todd. Perhaps while Grayson was Robin with the Titans, Bats got Jason, that ended well, Jason came back prematurely aged by his time in the Lazarus Pits, and thus calls Grayson kid.