Larson said he went ashore and called 911 to tell authorities he believed he had found the remains of 1-year-old Bryeon Hunter.
Cook County Sheriff Tom Dart said the body was significantly decomposed, but he was optimistic that the remains would be identified Wednesday as those of the missing child.
The body was taken to the Cook County medical examiner's office.
The boy's mother, Lakeshia Baker, 22, told investigators that she and her boyfriend, Michael Scott, beat her son with a belt and plastic clothes hangers April 15 before leaving him to die on the bathroom floor of their Maywood apartment, authorities said.
The couple initially reported that the child had been kidnapped, police said.
Investigators quickly determined that the kidnapping did not occur. The couple later confessed to disposing of the body but gave conflicting stories about where it was put in the river and by whom, according to authorities, who searched the Des Plaines on repeated occasions.
Baker and Scott each have been charged with murder. Baker was ordered held without bail, and Scott, 21, was held in lieu of $750,000 bail.
Larson, who operates a K-9 training center in Westchester, also used two of his dogs early in the effort to find the body.
Larson said he has searched the river every day since April 17 and grew emotional as he spoke with reporters.
"I remember looking over the bridge into the water last month and thinking, 'This can't possibly be the final resting place for this boy,'" he said. "I could not accept that."
Dart praised Larson for his perseverance and dedication in searching for the victim.
"He's been out here virtually every day," Dart said. "We're very thankful to him."
Freelance reporter Joe Ruzich contributed.