Police, alerted by store security, found the boy asleep in the vehicle in a Kmart parking lot at about 1:30 a.m. Friday.
They forced their way into the car and took the boy to the hospital as a precaution.
Meanwhile, they tracked the man to his Springfield home.
He told police he lost the boy while shopping, panicked and called someone else for a ride. The boy’s mother was working.
The 34-year-old man was not arrested and not immediately charged, but police say they expect to charge him with reckless endangerment to a child.