No, Nick Zano has not been recast. Instead, his character's grandfather, the Golden Age Steel or Commander Steel has just joined the show.
One of the earliest members of the Justice Society of America, Henry Heywood was a World War II soldier who was damaged during combat and had several mechanical devices installed to compensate for his injuries (think The Six Million Dollar Man). His grandson, Nate Heywood, was a former football star who had a gruesome injury that resulted in his leg's amputation. A freak accident caused by a group of super-villains triggered his latent metahuman gene and Nate became a being of living steel. Unable to control his powers due to his inability to feel, Dr. Mid-Nite and Mr. Terrific designed a "second-skin" costume that restricted his superhuman strength to manageable levels. Just how much of this comic book lore will be followed on Legends of Tomorrow remains to be seen.
Though we don't know in what capacity and to what extent, iZombie alum Matthew MacCaull (Star Trek Beyond, Tomorrowland) will play the elder Heywood.
Legends of Tomorrow returns Thursday, Oct. 13 at 8/7c on The CW.