I don’t follow it as closely as some others may but if I recall correctly there was an episode where Clark had to tell Pete. I think he needed Pete’s help to get back his little baby spaceship that someone had got hold of. I don’t remember the detail, or which season it was but he definitely told Pete.
Not “someone”: it was Pete who had hold of the spaceship. He found it after the tornado; Clark finally broke down and told Pete his secret because it was the only way he could convince Pete not to go to the press.