No one knows what their son is suffering from.
All doctors can tell Mr Lee Chee Keong and his wife Ann is that Yihong has developmental delay and that the cause is unknown.
Yihong is five, but verbalising his thoughts is still foreign to him. He communicates with a mix of body language and a handful of words that he knows, like "there", "banana" and "umbrella".
The couple said they have to observe his body language and behaviour to understand what he's trying to say.
Mrs Lee, 40, said the first two years after she found out about her son's condition were tough and she would break down when talking about it.
"Sometimes I would wake up in the middle of the night hoping it's a dream I could wake up from," she said quietly.
What made things worse was having to endure insensitive comments from others, sometimes unintentional.
"One of my good friends told me that when she's angry with her own kids, she will think of me and feel fortunate.
"I don't hold it against her because I know she didn't mean it, but I was very hurt," said Mrs Lee, who has another son, who is eight.
Going out with Yihong can lead to unwanted attention sometimes.
Resting on his nose bridge is a pair of prescription glasses to correct his short-sightedness - a refractive error of -8.00 dioptres in each eye, or what is more commonly known as 800 degrees.
Mrs Lee recalled an incident when a woman stared at Yihong's glasses, before signalling for "a whole kampung of people" to do the same.
Mrs Lee had her own witty comeback to the woman.
"I told her that Yihong is wearing glasses because he reads a lot and will grow up to be a professor. She walked away. I think she was embarrassed," she said, managing a laugh when recalling it now.
Mr Lee and his wife found support from other families after they joined the Rare Diseases Society Singapore a few years ago.
But despite the years that have passed, Mrs Lee still remembers the day when the first signs of trouble began showing during her pregnancy. They were told there was "excess water at the neck area" of the foetus.
This collection of fluid is indicated by nuchal translucency, said Dr Tony Tan, an obstetrics and gynaecology specialist at Raffles Women's Centre.
It refers to the thickness of the fluid at the back of a developing baby's neck. (See report at right.)
The couple, however, did not worry unduly as they were told that only a small percentage of babies would have problems.
Then, when Yihong was two months old, he suddenly stopped sucking milk, almost as if he forgot how to.
The situation was so dire that their doctor suggested tube feeding, something Mrs Lee adamantly refused to do due to the possible complications that come with tube feeding, like infections.
She tried to coax him into drinking small amounts of milk whenever he cried, indicating he was hungry, by cradling and rocking him in her arms.
A few months later, they realised that their son, then four months old, could be slower than his peers when they realised he could not lift his neck.
Yihong spent his growing up years crawling, until a year ago, when he took his first step without support.
Speaking to The New Paper from their three-room flat, Mr Lee, an engineer, said doctors have done blood and even chromosomal tests on Yihong, but the results were negative.
"(The doctors) recommended us to take Yihong overseas for more tests, or to go for CT (computed tomography) and MRI (magnetic resonance imaging) scans," he said.
Mrs Lee, an administrative executive, said: "When I asked the doctors if Yihong will get better by going through these scans, they said no. The tests would just identify the cause.
"I read that there is a lot of radiation that comes from CT and MRI scans. His brain is already affected. Why does he have to go through all this?"
Mr Lee, 41, had tried to be positive and had hoped his son would be able to attend a mainstream school, but would just be slower than the rest.
The couple soon realised that Yihong would have to go to a special school.
He now goes to Rainbow Centre at Margaret Drive five days a week. It is a school for infants, children and youth with a diverse range of special developmental and learning needs.
The couple have learnt to take pride in every little milestone that Yihong achieves.
"I think we are also learning from our son every day to not take things for granted," Mrs Lee said. At this point during the interview, Yihong walked towards his parents with a shoe in his hand.
"It means he wants to go out. He loves going out," said Mrs Lee, who looked at Yihong lovingly before taking him tenderly into her arms.