Brock was among the legion of "future kids" harvested and brainwashed by Head Alien II in his plot to destroy Joyce and Walky. He was one of the first future kids sent out against Walky and company, largely because of his disturbing background.
Brock wasn't the only one of Professor Doc's offspring to appear in Joyce and Walky! - another of his future children popped up when D.J. Wilcox attempted to rescue Bobby Walkerton. This particular one was the product of Professor Doc and a female clone of, uh, himself.