Board Thread:Questions and Answers/@comment-1038387-20131210001641/@comment-1038387-20131214231158

I'll give you some clues:
 * it's not revealed in his own title, but it is in a Milestone comic
 * some time after Worlds Collide
 * it's in a comic that appeared in a trade paperback (and for Milestone, that narrows it down considerably)
 * we have a page on it.