"It didn't look like there was any foul play from what I saw," he said.
An autopsy was scheduled, with the cause of death to be released Monday.
Hidalgo County Sheriff Lupe Trevino, who called the death "suspicious," said he would wait for an announcement on the cause before deciding whether to file charges.
The child was believed to have been dropped off by his mother to spend the day with the bedridden relative, The McAllen Monitor reported Friday.