WAUKEGAN, Ill. (CBS) — A 26-year-old man has agreed to plead guilty to killing his ex-girlfriend’s infant son last summer and tossing the body in a Dumpster.

WBBM Newsradio’s Steve Miller repors Demetries Thorpe agreed to plead guilty to first-degree murder in the death of 5-month-old Joshua Summeries in exchange for a guarantee he’d face no longer than 35 years in prison.

Thorpe was arrested last August and charged with first-degree murder. Prosecutors said he suffocated Joshua after the baby would not stop crying at the home Thorpe shared with his now-former girlfriend in Zion. He claimed he disposed of the body in a Dumpster, but Joshua’s body never was found.

Lake County State’s Attorney Mike Nerheim said he believes the plea agreement is appropriate, even without having found Joshua’s body.

“We believe that, based on his taking responsibility for his actions and a number of other factors we considered, that this is an appropriate disposition,” he said. “I really don’t think that would have changed had we had a body.”

Nerheim said he believes Joshua’s mother also is satisfied with the plea deal.

“Nothing is going to give her her son back, but the decision to enter this plea, she was part of that decision-making process; the decision in terms of what the actual offer and the ultimate disposition was going to be,” he said. “From the beginning, she was part of that process.”

Thorpe has been in custody since his arrest last August.

No trial date had yet been scheduled.