The judge at Reading Crown Court sentenced the father to six years behind bars for child cruelty and indecent acts with a child.
His girlfriend, who slept with the boy regularly, was sentenced to nine years for child cruelty, indecent assault and indecent acts with a child.
Judge Maria Lamb heard how the victim first told police of the abuse he was suffering in 1998 but his father, now of Weymouth, Dorset, lied about it.
The victim, now in his 30s, continued to fight for justice until the couple were charged with a string of sex offences.
When he was finally arrested, the callous dad told cops he was trying to keep his son on the "straight and narrow".
Prosecutor Ruby Silva told the court who the couple, now at least 60, forced the boy to watch them have sex over several years in the mid-1990s.
She said they would also touch the boy's genitals and force him to perform sex acts on the woman to "educate him about sex".
The abuse is believed to have started when the dad took his son from his first marriage and moved him into the home he shared with his new partner.
Shaun Smith, counsel for the step-mother, admitted she had consistently denied any wrong doing right up until the sentencing.
Judge Lamb said: "This was a child who had been neglected by his mother and was particularly vulnerable.
"You have both argued that this whole trial process has weighed heavily on the both of you. This will provide you absolutely no mitigation.
"This was your own warped sense of right and wrong. The damage that you two have done is unacceptable".
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