As a boy of few words, he rarely expresses his love for his parents.
"Hoan tried his best to save money for us. In his first high-school year, he skipped breakfast all the time, and only spent VND15,000 (US$70 cents) for a meal. I had to tell him to at spend at least VND20,000 (US$90 cents) to keep health and study. Your dad and I can handle it," said Hoan's mother Nguyen Thi Thach.
About an hour by bus across the Red River from Hanoi city centre - and far from Hoan's school - his parents are working to build a house from scratch. But it is not for them. They have worked as construction labourers in the capital for the last two years to earn enough money to send their son to high school.
Hoan passed the entrance exams to three high schools for gifted students after graduating from junior high school in the summer of 2012. One of them was in his peaceful home in Thai Binh province. The other two were the Hanoi National University of Education and Hanoi University of Science (HUS).
"I chose HUS high school because many students of its students have won international prizes. The world renowned mathematician Professor Ngo BaoChau was also a former student there," Hoan said.
Three years ago, Professor Chau won the prestigious Fields Medal, the mathematics equivalent of the Nobel Prize is his inspiration for studying Math, Hoan said.
While totally committed to supporting Hoan to study in Hanoi, his parents could not help but worry about the financial problems they would have during the next few years. They decided to leave the fields and go to the city to work - and to be together with their son.
"Jobs at construction sites also provide meals and a place to sleep, so we could earn and save more money than working back home," said Thach.
Thach's round face made her look younger than her age of 38, but little freckles on her face and her dirt-stuck nails reveal her job as a heavy labourer.
The money was better she said, but the wage for nine hours a day working at the construction sites was only VND130,000 (US$6), just enough for a venti Starbucks frappuccino.
"We could not do anything on rainy days. That means we had no money," lightly smiled Thach.
Hoan's parents would average about VND5 to 6 million ($240-288) between them per month, almost two thirds of which went to pay for Hoan's expenses - and another VND1 million ($48) to send back to his brother in eighth-grade back in Thai Binh.
Looking back, Hoan shyly puts his head down. "I was so childish at that time. No breakfasts, had instant noodles instead of proper meals. I thought that if I ate less, I could save more money".
It was hard enough in the beginning, but Hoan has never given up. "I thought that if I had the chance to study in Ha Noi, I would never give up," he said. To him, studying is the only way to climb out of poverty and rise up in life - and to make it worth all his parents' hard work.
"Hoan is friendly and open, but he is also very independent", said Vu Tien Sinh, his dormitory roommate. But to Hoan's mother, his independence sometimes broke her heart.
Thach told how her son was sick from food poisoning for three days and he did not even call her. It was not until a friend of his let her know to come by his school and take care of her son. "Hoan never wants to worry us," Thach said.
Two years of sweat and tears was all paid off, not only with the the Olympiad gold medal, but with the respect he has earned from his peers and seniors.
"There is something special about Hoan. He always comes up with new ideas to solve unfamiliar maths problems. Whenever someone asked who in your class can go far in the future, I told them about Hoan," said Hoan's form teacher-cum-Olympiad team trainer Pham Van Quoc.
Thach could not hide her laughs when she remembered how vague she and her husband were when they knew their boy won the medal. They were too happy to do anything else.
"It is ok for us to work a little harder so that our children can have a better life than their parents'", Thach lightly said.
For the parents, having to move around finding a new job about six or seven times in two years was no big deal, as long as their boys can study for a bright future.
"Sometimes the owners of the houses we built did not allow us to sleep inside the building. We set up a small tent next to the construction site. When storms came, we had to use nylon to cover the roof, otherwise we would be wet all night," the mother said.
At least it has been better now as she and her husband were spared a corner in the building they were working at, even though all they have are some neatly folded blankets on a simple wooden bed.
"I gave the whole VND15 million (US$720) that the Ministry of Education awarded me for my Olympic medal to my mom, asking her to keep it so that we have some savings," said Hoan.
The boy is trying his best in his last year of high school to achieve his important goal, getting a full college scholarship abroad.
Apart from attending school in the morning, he also takes English classes in the evening, twice a week, to enhance his English level. He is thinking of going to America, Canada or Singapore etc., anywhere that offers him the chance to continue studying Math.
"He wants to study at the a-arduniversity or something like that in the US," the mother beamed a warm smile.
She is dreaming the dream that her son would fly far to another horizon, and that he would one day come back to the place he was born.
"No place is like home. I just wish that my family can stay together forever," her eyes were twinkling.