The boy's father, who allegedly was driving the truck while under the influence of alcohol or drugs, is being investigated for vehicular assault, said Shari Ireton, spokeswoman for the Snohomish County Sheriff's Office.
The 62-year-old Arlington man was arrested at the scene on Sunday and later released so he could be with the child at the hospital, Ireton said. The boy on Tuesday remained at Harborview Medical Center in Seattle.
The man's truck has been seized as part of the ongoing investigation by the sheriff's collision detectives.
Under state law, passengers who are under 16 years old must wear a seat belt or the driver can be cited.
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