The boy's mother, Elizabeth Johnson, 23, says she gave her son, Gabriel, to complete strangers.
"They wanted the baby, and she simply gave it to them," said Bob Johnson, Elizabeth's grandfather.
Bob Johnson said he doesn't understand the decision, but says he believes his granddaughter's story. She said she gave her baby to a Texas couple she met while on the run from police.
"It sounded like a pretty weak story, almost on the border of unbelievable, that a person would do that to 8-month-old child," Bob Johnson said.
Elizabeth lost custody of her son after she failed to show up for a custody hearing between herself and Gabriel's father.
There has been no sign of the boy since the day after Christmas. Elizabeth initially told the boy's father she killed their son, but her grandfather says he believes the child is safe.
However, he admits he has no clue where the boy is.
"I don't think she'd harm the baby. I'd be surprised if it turned out that way," he said.
On Tuesday police found Elizabeth's car outside a San Antonio Hotel. Police processed the car for any clues that might lead them to the missing boy.
