Sixty soldiers and three sniffer dogs have been added to the hundred or so people, including firefighters, police and local residents, searching for the boy, who left the family’s holiday home on his bicycle at 6.00 pm on Saturday.
His bike and one of his shoes have been found.
Winds of up to 120 kilometres hit the south of France at the weekend.
The wind and six-metre-high waves have hampered the search, as has the thick vegetation on the island, where motor vehicles are banned.
“Weather permitting, we will begin investigating at sea with boats on the south-west of the island [where the bike and shoe were found],” regional official Paul Mourier said on Sunday evening.
Pierre’s father is British and his mother is French.
He and his two brothers and sisters had just arrived for a holiday on Porquerolles.
About 350 people live on the 12.5-square-kilometre island just off the Provençal coast throughout the year.