Charges of sexual child abuse, second-degree sex offense, sodomy and unnatural and perverted sexual practices were dropped in exchange for the plea.
The defendant's name is not being used to protect the identity of the child.
Now 14, the boy was 11 when the defendant began fondling his genital area, first outside his clothing and then inside, court records said.
The touching, which always occurred when the mother wasn't home, progressed to a variety of sexual acts, court records said.
"This was repeated sexual abuse over a two-year period," said Assistant State's Attorney Andrew Kramer. "I can't imagine an 11-year-old having to come home from school and have to deal with that nightmare."
Kramer said he was concerned that the defendant would return to the home once released.
"There was a significant violation of trust here that is not to be taken lightly," he said.
The defendant, who had lived with the child's mother for nine years when charged in September 1996, told police the abuse began when the child asked him questions about sex after watching pornographic movies.
McDowell ordered no contact with the victim during the three-year probation period after the defendant is released from prison.
"This young man will never be the same," McDowell said, noting the victim is now in counseling.
The victim's mother told McDowell Tuesday that she doesn't want the defendant back in the home she shares with the victim and her four other children.