Parents found the boy in "the car outside their home", the source said, while civil defence confirmed his death in the town of Saint-Gratien in the Paris suburbs.
It comes just days after siblings aged two and four were found dead in a car in Carpentras in south-east France.
Although the investigation is ongoing in that case, police say that they believe the children wandered off during a family visit and got into the car, then becoming trapped.
France has been scorching under an extremely intense and long-lasting heatwave for the past week.
Prime minister Sébastien Lecornu said on Tuesday that at least 40 people have died from drowning since the heatwave began.
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