and wandered confused and alone in the bush for 17 days before local hunters guided him to Monguno, 137 kilometres by road from Maiduguri.
Gaal said he met the man in Monguno on Friday and had planned to visit him again this week "but unfortunately Monguno was taken" by Boko Haram on Sunday.
"We don´t know where he is now," Gaal told AFP.
The MSF official said mothers on the run were afraid their sons would be conscripted to join the vigilante groups who are helping the military fight Boko Haram.
One woman in Monguno "was dressing her son as a girl to keep him from being drafted", Gaal said.
While some mothers are taking measures to protect their sons, a growing concern is the number of children arriving in camps without parents, said Abubakr Bashir Bakri, MSF´s head of mission in Nigeria.
"There are many of them... We are concerned about their psychological state," he added.
One woman who escaped, Saudatu Isufu, said Monguno was gripped by "a humanitarian crisis" but the town was currently inaccessible to aid groups following the Boko Haram takeover.
"People trapped there need to be rescued," she told reporters in Maiduguri.
Those who escaped needed to pass a security check before being allowed to enter Maiduguri, with the military worried that insurgents would try to infiltrate the state capital by blending in among the IDPs.
Abdulkadir Ibrahim, of Nigeria´s National Emergency Management Agency (NEMA), described the group that reached Maiduguri as "disturbed and hungry" and said rescue workers were trying to stabilise them.
Lack of relief staff on the ground has been a chronic problem in the northeast. MSF and other organisations have said they would like to reinforce their presence, especially in Maiduguri. "The increasing number of people puts pressure on us and requires a lot of resources," Bakri said.
But with violence escalating and attacks impossible to predict, sending large teams to Borno was likely to be unrealistic in the short-term, he added.