ALFRED – A Biddeford man charged in the death of an 11-month-old infant he was baby-sitting two years ago pleaded guilty Thursday to a lesser charge of assault on a child.

In a plea agreement in York County Superior Court, prosecutors dropped manslaughter and aggravated assault charges against Shawn Small, who was accused of shaking Chason Treadwell on June 15, 2002. The baby died from brain injuries two days later.

Small was originally charged with aggravated assault, but the charge was later upgraded to manslaughter.

Under the plea agreement, Small will serve a year, nine months and 20 days in jail for assaulting a child under age 6. He already has spent about a year behind bars, according to prosecutors.

An autopsy determined that the baby died as a result of lack of oxygen to his brain. The baby had suffered additional injuries, including broken ribs, believed to have been caused by earlier shakings.

At the time of the incident, Small was dating the baby’s mother, Dawn Treadwell, who lived in the same building on Wentworth Street. Small had two children of his own and occasionally baby-sat Chason, police said.

Small called police to report that the infant was ashen and lethargic. By the time police arrived, a bruise was spreading across Chason’s temple.

Small allegedly offered several different explanations, including that the child hit his head on a box of diapers and he might have hit his head on a door frame. Small later claimed that he just didn’t remember how it happened.

The Department of Health and Human Services took Small’s two children, a 4-year-old girl and a 2-year-old boy, into custody at the time of his arrest.