Witnesses say the boy went into a waiting ambulance on Monday, part of a group of boys and their soccer coach trapped in the cave since June 23. Last Monday they were discovered huddled on a narrow rock shelf deep within the flooded cave system.
The risky bid to rescue the boys - aged between 11 and 16 - resumed after a break on Sunday night to replenish oxygen supplies and make other preparations deep inside the cave complex in northern Thailand's Chiang Rai province.
The second day of the underground rescue attempt began at 11:00am local time (2:00pm AEST).
The ambulance drove toward a helipad, where a helicopter was seen taking off shortly after to the cheers of the crowd below.
Thai officials have been tight-lipped about the rescue operation, and would not comment on how many people were removed Monday. Four more boys and the coach remain in the cave.
Narongsak defended the lack of public disclosure of the names of the boys who had already been guided to safety, saying it "will create ill feeling" if their names are released while others languish inside the cave.
Unconfirmed reports that the boys' coach, Ekapol Chanthawong, 25, was among the first four to be evacuated because of the poor state of his health, were dismissed on Monday.
Rescuers have been unable to extend a hose pumping oxygen all the way to where the boys are, but have brought them some oxygen tanks.
Once that task is complete, the boys are then handed over to a team of specialist who are assisting them through the remainder of the cave, much of which they can wade through.
Authorities have been rushing to extract the boys, ages 11-16, and their coach from the cave as the annual monsoon bears down on the mountainous region in far northern Chiang Rai province.
On Monday, skies were largely clear over the site but rain has been forecast for at least the next three days.
Tropical Storm Chris strengthening, Tropical Storm Beryl moving towards the Caribbean
Elsewhere in the tropical Atlantic, Beryl - once a hurricane - has weakened substantially on approach to the Lesser Antilles . As the third named storm of the 2018 Atlantic hurricane season , Chris developed more than a month ahead of normal.
The boys brought out of the dark, winding cave on Monday are "safe and conscious", said Chiang Rai province acting Gov. Narongsak Osatanakorn, who is in charge of the rescue effort.
The boys will "be evaluated by doctors for five to seven days", Dr Chokedamrongsook said.
"This morning they said they are hungry, and they wanted phad kra pao", he said. Two more were taken from the cave before nightfall.
"Everything is all good - weather, water and the boys' conditions", he said, adding, "We will speed up [the operation]". Visitors would need to wear sanitized clothes and stay two meters away from the children.
A spokesman for Musk's Boring Co. said Sunday that the company has four engineers who are "offering support in any way the [Thailand] government deems useful".
Elon Musk has offered his assistance and has floated plans to save the soccer team stranded in a Thai cave network. Secondly, after a delay of several hours required them to replace the oxygen tanks along the path, the teams of divers are going back in, to bring more of them to the surface.
The worldwide effort to save the group has paired divers in "buddy teams" with the remaining boys and their coach. Two other boys came out at 7:40 p.m. and 7:50 p.m.
Rescued schoolboys are moved from a military helicopter to an awaiting ambulance at a military airport in Chiang Rai, Thailand, July 9, 2018. They got trapped in the cave after a sudden spell of heavy rains blocked the entrance of the cave.
"We'll start preparing for the next rescue operation, and we believe we'll do better than today and it will be highly successful", the rescue mission chief said.
Thai premier Prayut Chan-O-Cha was also due to visit the rescue operation late on Monday.