Raymond Pryer, 3, was found inside a Discovering Me Academy van in northwest Harris County.
Raymond was left alone for at least four hours last July after a group returned from a field trip. Temperatures inside reached at least 113 degrees, Constable Alan Rosen said.
A medical examiner has now ruled his death to be a homicide.
Houston police say the district attorney's office is aware and this case will go to a grand jury.
The day care was shut down permanently last month when the state revoked their operating license.