Police did not identify the boy, who was expected to survive after being treated at Thomason Hospital, officials said.
"It's bizarre. I've never seen anything like it," said El Paso police spokesman Darrel Petry. "A witness saw something come out of the car, and they saw something move."
Investigators believe De Leon was passing through El Paso on his way to California when the incident occurred on a highway access road. Police said De Leon was traveling with the boy and a 4-year-old girl.
The girl was not harmed, and police declined to describe the relationship between the children and De Leon.
De Leon remained in the El Paso County Jail on Sunday with a bond set at $100,000, detention officer Edward Sustaita said. Authorities could not say whether or not De Leon had an attorney.
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