A man already serving time in a Pennsylvania prison has pleaded guilty to killing a convicted ax murderer's 3-year-old son in 2003.

WFMZ-TV reports (http://bit.ly/1qMKcSh ) 51-year-old David Cummings entered the third-degree murder plea Thursday in the death of his stepson William Spayd. He was sentenced to 15 to 40 years behind bars. He's been incarcerated since pleading guilty to burglary and arson in 2004 and won't be eligible for parole until he's 83.

William's biological father, Chester Spayd, killed his own mother's ex-boyfriend with an ax in 2000.

Berks County authorities say Cummings told detectives that William kept poking him while he was asleep and he punched the child in the forehead, knocking him into a dresser. Police say Cummings told a 911 dispatcher William wasn't breathing.

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Information from: WFMZ-TV, http://www.wfmz.com