Richters also agreed to testify against the boy's mother, Melissa Robitille, who police say approved giving her son, 13-year-old Isaac Robitille, vodka through the intravenous tube because the child was "being fussy."
He pleaded guilty on Wednesday to a manslaughter charge and 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.
Prosecutors say Isaac was blind, had no ears, a cleft palate and developmental delays.
Richters, who has been in prison since his arrest last year, can argue for less prison time at his sentencing.