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