His boy is one of 12 footballers and their coach who have been trapped in the flooded cave in northern Chiang Rai province where heavy rains have hampered the harrowing search.
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Weeping relatives spend much of the day praying and have been joined by chanting monks and Christian well-wishers singing gospel and reading Bible passages.
They are working hard to keep spirits buoyed as the desperate search drags on.
"I am very worried," the 55-year-old father of Mongkol Boonpiem told AFP from the mud-slicked site close to the Laos and Myanmar borders.
Thinnakorn told AFP his son is a "good boy" who loves to study -- almost as much as football.
He joined the local "Boar" youth team a year ago and on Saturday went to practice like he has done many times before.
He didn't tell his dad he planned to go trekking in the cave, and his family started to worry when he never came home.
"I learned that he was trapped at around 9 p.m. that day,"
"Since then I've been here in front of the cave," the grim-faced father of two said.
Three British divers and US military personnel have been deployed to the scene to help hundreds of Thai NAVY seal divers, soldiers, borders guards and police in the daunting search for the boys.
The 10-kilometer (six-mile) cave is one of Thailand's longest, and has a notorious reputation for being one of the toughest to navigate, especially in the monsoon season from July to November.
But the adventurous young team knew the cave well and had visited many times before. Some of their teammates had even surveyed the site on previous visits, officials said.
Search teams found signs of the team when they entered the cave: their backpacks were near their entrance, along with backpacks and football boots.
Further in, rescuers found footprints and handprints believed to belong to the missing 13.
'Cheerful boy'
As rain continued to pound the area Thursday, relatives clung to hope the youngsters were still alive.
Some prayed at Buddhist shrines of bright yellow marigold flowers, food offerings, dolls and incense sticks.
"We all are sad and hope to see him soon," said the great aunt of 11-year-old nephew Phanumas Saengdee, nicknamed Mick, is among the missing.
She has been sitting vigil with Mick's mother and grief-stricken grandfather who has barely spoken in days.
"I'm taking care of his Grandpa... he hasn't eaten much. He was the one who raised Mick, so I'm trying to comfort him," she told AFP, declining to be named.
The dramatic, days-long rescue has captivated much of Thailand and prompted emotional messages of solidarity from football clubs across the country.
Some players formed the number '13' on football pitches while others joined hands and made a heart figure, according to aerial photos circulating online.
Thailand's top Islamic body also called on Muslims to pray for the missing boys at Friday prayers across the country.
Football is one of the most common pastimes for young boys in Thailand, where the sport is wildly popular.
The Boar's Facebook page shows photos of the young players in red football shirts flashing peace signs and smiling, while others show teammates holding up a trophy.
Some, like 13-year-old Sompong Jaiwong, who is trapped in the cave, joined youth leagues with the wish of one day going professional.
"Pong is a cheerful boy, he likes football, and every sport. He dreams of becoming a footballer for the Thai national team," the teenager's teacher Manutsanun Kuntun told AFP, using his nickname.
"Our class is still confident he will be OK," she said.
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