Robert Bloch 
This Crowded Earth 

Harry Collins is an ad-executive in a future Chicago on an Earth whose population has exploded beyond imagining. Crazed by the pressures of overcrowding, he seeks escape with a suicidal leap from a skyscraper. Stopped, he is hustled off for psychiatric treatment in a odd encampment where he meets an falls in love with an accommodating nurse named Sue. But who is the strange Dr. Leffingwell, performing experiments on the premises? Harry’s horrific discoveries in the secret lab cause him to flee into the outside world — flee into the forces that would help change and shape this tortured world. But then, years later, when his assassin’s rifle is trained on Dr. Leffingwell himself, he is halted by the mutant product of that fateful lab. His own son.