Randy Fowler also reveals how he almost killed their abusive and racist dad - who he nicknamed The Creature for his inhuman behaviour - after he had raped Randy over 50 times when he was only 14.
The book was published just weeks after Oscar-winning Spacey faced a slew of sex allegations from men - some of who were only teenagers at the time of the alleged assaults.
Now in Randy's first interview since the explosive book came out the 62-year-old, told The Sun Online: "I heard Kevin checked himself into some sex addiction clinic - well excuse me but you're in the wrong building. It's far more serious than that.
"Kevin was a king on his throne but he has now been dethroned by all these allegations; taking advantage of people using his wealth and power.
"All my life I thought I had protected Kevin from my father and it turns out it's all for nothing because he's worse than my father, The Creature.