The boyfriend of a woman whose 5-year-old son was found dead in a Massachusetts state park has pleaded guilty to manslaughter in the boy's death and sentenced to decades in prison.Joseph Stapf pleaded guilty to manslaughter, second-degree assault, falsifying physical evidence and tampering with a witness. He was sentenced to 22 to 45 years in prison in total.The body of Elijah Lewis was found buried in a Massachusetts state park last year.>> Watch: Victim impact statements | Stapf apologizes | Judge's commentsIn court, prosecutors outlined disturbing details of systemic abuse Elijah allegedly suffered while living with his mother, Danielle Dauphinais, and Stapf, along with the couple's 2-year-old child. A series of texts between Dauphinais and Stapf were read in court in which they talked about abusing Elijah and not feeding him."He suffered instances of assault, was denied food and water, was isolated in a room sometimes without clothing and blankets," said Assistant Attorney General Bethany Durand. "He was left alone in that room to stand in one spot for hours at a time."At one point, Stapf texted Dauphinais that they needing to feed Elijah and let him sleep so he would look better so they could go out and do things, but Dauphinais responded that she didn't want to do that, according to prosecutors."He said he wants food and he wants me to stop starving him because it's not nice," Durand said, reading one of the texts.Prosecutors said that in one incident, Stapf picked Elijah up by his arms, yelled at him and threw him on a bed. Elijah struck his head and was injured, prosecutors said.Prosecutors said that as the abuse went on, Stapf at one point began urging Dauphinais to stop what he referred to as "torture," but she refused.In court, prosecutors said Stapf returned home to find Elijah in a tub in the home's basement, naked and bleeding from an injury to his head, with broken tiles around him. Stapf put a bandage on the boy and took him to bed but didn't seek medical help, prosecutors said.Elijah died at some point in the following days, between Sept. 21 and Sept. 24, 2021.As the details of the abuse were read in court, Stapf kept his head bowed, looking at times over at the prosecutor.Prosecutors said Dauphinais allegedly told Stapf to get rid of Elijah's body. The two allegedly put Elijah in the back of a pickup truck and drove to Massachusetts, where they buried the boy in a shallow hole in a state park in Abington.Prosecutors said the two then abandoned the truck at the Mohegan Sun casino. They were later arrested in New York.Elijah's father, Timothy Lewis, called in from Arizona to speak to the court during sentencing."You are less than human," Lewis said. "You could have stepped up to stop this at any point if you had been man enough. You could have brought him back to us at any point."Stapf spoke in court before the sentences were handed down."I wish I could go back and change everything, and I wish I sought help. Elijah deserved to be loved. He didn't deserve to go through that," Stapf said. "I'm so sorry. I truly am. I wish I could take it back. I'm sorry."Dauphinais has been indicted on one count of first-degree murder and one count of second-degree murder in her son's death. She has pleaded not guilty.