The victim, identified as 20-year-old Brian Nevins, was found Saturday afternoon outside a residential facility in Langhorne, Pennsylvania, that provides support and services to people with severe disabilities.
Middletown Township police say Nevins was left in the van on the Woods Services campus for five hours after a trip to Sesame Place.
Nevins, the son of a retired New York City police officer, had previously lived in Queens. The Bucks County coroner says he died of hyperthermia.
The driver has been suspended, but so far no charges have been filed.
The investigation is ongoing.
In November, a 17-year-old Woods resident died when he was struck by at least two cars after falling or jumping from a highway overpass. That death was ruled accidental.