Ricky Chavis, 41, was convicted of the lesser offense of false imprisonment. He was sentenced later Wednesday to the maximum of five years in prison.
Jurors deliberated about three hours. Chavis, who was convicted in the 1980s for molesting two other boys, covered his face with his hands when the verdict was read.
Chavis was charged with having sex with Alex King, when the boy was 12, in the months before he and his brother Derek killed their father on Nov. 26, 2001.
Kelly Marino, the mother of Alex, now 13, and Derek, now 14, was visibly disappointed as the verdict was read.
"It's an outrage," she said later. "It's insane."
Assistant State Attorney David Rimmer also said he was disappointed, adding, "I'll just take the jury's verdict."
Chavis had faced a possible life sentence on the kidnapping charge. Florida law permits a kidnapping conviction if the victim is under 13 and confined without parental permission for criminal purposes.
Circuit Judge Frank Bell cited Terry King's frantic search for his children, who had run away from home in the days before the slaying, in imposing the maximum penalty for false imprisonment.
"His friend, Mr. Chavis, is supposed to be helping him, but instead of helping him he's the one that's got them," Bell said. "That's a little hard to swallow. That's unconscionable."
The state's case rested heavily on Alex King's testimony that he and Chavis had oral sex in the months before the killing.
"He said that he loved me," Alex testified Tuesday.
Prosecutors said that 10 days before the killing, the brothers had run away from home and stayed with Chavis, who lured them with video games, marijuana and pornography. Alex testified Chavis told him if the boys killed their father they could live with him and that he hid them from police afterward.
The defense argued that the allegations were part of a plot to frame Chavis for the murder of 40-year-old Terry King so the boys could avoid life sentences.
"These are psycho-pathological killers," defense lawyer Michael Rollo told the six-member jury. "It was a dreamed-up story to make Mr. Chavis a target so that Alex and Derek King could be acquitted of murder."
He had urged jurors not to be swayed by the boys' appearance: "The cuter you are in this society, the better it is for you."
Chavis is facing another trial Feb. 24 on charges of accessory after the fact to murder and evidence tampering.
The boys and Chavis were tried separately for first-degree murder. One jury acquitted Chavis while another convicted the brothers of a lesser offense: second-degree murder without a weapon.
A judge threw out the boys' convictions and ordered mediation that resulted in both pleading guilty to third-degree murder. Alex is serving seven years and Derek eight at separate juvenile facilities.
By Bill Kaczor