Another five Tunisians are still missing following Thursday night's attack that killed 84 people in the southern French resort city and was claimed today by the Islamic State jihadist group (IS).
The ministry said the boy was found dead in hospital. His mother was among the other three identified Tunisian victims.
Nice is just an hour's flight from Tunis, and the city is home to many Tunisians.
Mohamed Lahouaiej-Bouhlel, 31, the driver of the truck that ploughed into a Bastille Day crowd in the city, was himself of Tunisian origin, from Msaken near Sousse.
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It also said it was opening a judicial inquiry into the attack.
Tunisia, like France, has faced a series of jihadist attacks in the past year, killing dozens including 59 foreign tourists.
The country has been under a state of emergency since November 2015.
Tunisian and French citizens are among the top nationalities who have joined jihadist organisations including IS.