Parents Accused of Murdering Obese Son Fed Him Junk Food Diet, Prosecutors Say

Casper O'Brien -- the obese 7-year-old who died weighing 255 pounds -- was fed a diet of French fries and chips ... this according to prosecutors who charged his parents with murder.

John Potbury, the Genesee County Chief Assistant Prosecuting Attorney, tells TMZ ... Casper's parents, Damien and Jessica O'Brien, fed him the same thing every day ... a large bag of potato chips, a large order of French fries, apple juice, and Sparkling Ice carbonated water.

According to Potbury, there's no indication Casper was allowed to exercise to shed weight ... and he says the child had bed sores from staying stationary in bed for great lengths of time.

We're told Casper weighed 104 pounds in February 2024 ... but, less than two years later, he'd gained more than 150 pounds.

Prosecutors say Casper's fingernails were uncut.

In addition to Casper's condition, Potbury says the home itself was in bad shape ... with layers of dirt covering things and a non-operational toilet filled with human feces.

As you know ... Damien and Jessica have been arrested and charged with second-degree murder, child abuse, and torture after Casper's death. Prosecutors say Casper was never enrolled in school, and he and his five-year-old sister essentially never "existed in the eyes of the government."