The planet Zwen rotates so slowly that the "nights" are over six Earth-months long. Consequently, the colonists of that world developed the ability to transform into a dormant stone state and hibernate for the duration of the sunless periods. when the sun returns, they thaw and reawaken.
Most Zwennites are perfectly content to stay on their home planet for their entire lives. But when Dag Wentim learned of other worlds, he wanted to visit them. Dag did not want to waste half of his life in a dormant state.
When Dag found out about the Legion of Super-Heroes, he thought that he stood a chance of joining them since he is almost indestructible in his stone state. Unfortunately, Dag's ability was considered too limiting because he was stationary in his stone form and it was difficult to communicate with him in that state.
However, Stone Boy soon met up with a group of other Legion rejects and they formed the Legion of Substitute Heroes.
When the Legion of Super-Heroes learned of the Subs' existence, they held a contest to reward one of the Subs with Legion membership. While all of the Subs did well in the contest, Stone Boy, who knowingly forfeited his mission in order to save a young girl's life, won. Elated, Dag nevertheless realized that he wanted to stay with his friends more than he wanted to be a Legionnaire.
When the Subs temporarily disbanded, Stone Boy found work as an intergalactic surveyor. Because of his ability to go dormant, he is quite cost-efficient, using less food, oxygen, and other supplies than a normal spacefarer.
Throughout most of his missions, Stone Boy could only transform into an inanimate stone statue and was often used in numerous ways such as being dropped on a villain from above by his teammates, or as a battering ram, or even as a distraction while the other "Subs" carried out their mission.
During the "Five Years Later" timeline, Dag underwent hypnotherapy and gained the ability to stay conscious during his transformation and, eventually, even to move in his stone state which was a major achievement for him. On occasion he has even been shown transforming only part of his body to stone.