His concerned parents, who knew he was being bullied and was fighting with other children, enrolled him in taekwondo classes.
Mr Singh said the instructor - who he only refers to as Mr N - offered him personal classes at his home dojo. He said his instructor has since died.
The politician does not go into detail of the abuse but says it soon began to feel "normal".
"That's the thing about abuse - it can make the victim feel an overwhelming sense of shame, a shame so disabling that one suffers in silence," he writes.
"I told no one, and I told myself not to think about what had happened. In a way, I prevented myself from actually accepting the truth."