If I remember correctly, he took his parents on a jaunt to a South Sea island - time travel may have been involved; I'm not sure - where they opened a treasure chest which contained an ancient virus which put the two of them into a coma and ultimately killed them both. This is revisionism, though, as the original account simply had them dying of old age, and had Pa Kent telling Clark on his deathbed that he ought to become Superman - which would tend to mess up the account of him causing both their deaths as SuperBOY. Like I said, Superboy really made a hash of Clark's original history.