ST. JOHNSBURY, Vt. » A Vermont man has admitted he killed his girlfriend’s severely disabled child by putting vodka in the boy’s feeding tube.

Walter Richters pleaded guilty Wednesday to a manslaughter charge in the Aug. 22, 2014, death of 13-year-old Isaac Robitille at the Hardwick home they shared with Isaac’s mother.

Richters, 38, is to be sentenced to at least three years in prison as part of a plea deal with prosecutors. A sentencing date hasn’t been scheduled.

Richters also agreed to testify against the boy’s mother, Melissa Robitille, who police say approved giving her son vodka through the intravenous tube because the child was "being fussy."

Prosecutors say Isaac was blind, had no ears, a cleft palate and developmental delays.

Melissa Robitille has been charged with second-degree murder. Her trial is scheduled for early next year.

Richters initially was also charged with second-degree murder and, under a proposed plea agreement, would have served 18 months in prison. But a judge last month rejected that proposal.

Richters, who has been in prison since his arrest last year, can argue for less prison time at his sentencing.