Ahmed, who is aged six or seven, used a mobile phone given to him by a British aid worker in a French migrant camp to warn her that they were running out of oxygen.
Volunteer Inca Sorrell was attending a conference in New York when she received his texts and contacted a colleague in Britain, Tanya Freedman, who alerted police.
Officers traced the phone and stopped the truck, taking all those inside into the care of immigration authorities.
“It's an extraordinary story. It's a global network of people who came together to save lives, led by this young boy,” Freedman, who works at British charity Help Refugees, told AFP.
Sorrell had met Ahmed in a children's centre in “The Jungle” migrant camp in the French port of Calais, where thousands of refugees and economic migrants live while waiting to travel to Britain, often illegally.
As the camp was being demolished last month, she gave the boy a phone with credit and her number on, with instructions to call her if he found himself in danger.
“He texted me to let me know that he'd made it to England,” Sorrell told delegates at the conference.
“Then he told me that he was stuck in the back of the lorry and the driver wouldn't stop, that it was a refrigerated lawyer and he had no way of getting out.”
The text read: “I ned halp darivar no stap car no oksijan in the car no sagnal iam in the cantenar. Iam no jokan valla.”
This means: “I need help, driver no stop car, no oxygen in the car, no signal, I am in the container. I am not joking, Wallah (I promise).”
A police spokeswoman confirmed a truck was stopped at a motorway service station in Leicestershire in central England on Thursday, and 14 people were arrested on suspicion of being illegal immigrants.
She said one man was arrested on suspicion of smuggling them into Britain, while “safeguarding measures were put in place for a child who was found in the truck”.
Ahmed was travelling with his older brother, estimated to be aged around 20.
Around 40 migrants a day were detained after crossing the Channel from France to Britain last summer, according to an official report by HM Inspectorate of Prisons.