The boy was 17 when the acts took place in 2010.
Intellectually disabled, he was a student at a special needs school in the east.
While going home after school one afternoon, he met Christian Bora Ganapathi, then 19.
A few days later, the two met again and exchanged phone numbers.
They then started speaking regularly on the phone and Ganapathi realised the boy was intellectually disabled.
The boy cannot be named because of a gag order protecting his identity.
On at least five occasions in August and September 2010, Ganapathi tried to masturbate the boy, or perform oral sex on him.
The sex acts took place in various locations including Ganapathi's home, a staircase landing of the block and shower cubicles in the Aloha Loyang chalet.
The sixth time they met, the boy was in Ganapathi's home, playing with a Nintendo game console.
Feeling aroused, Ganapathi removed the boy's clothes and his own.
He then tried to have sex with him, but stopped when the victim said it was painful.
The boy later told a school teacher about the incidents and a police report was made in late October 2010.
Reports from the Institute of Mental Health showed that the boy did not have the mental capacity to consent to these acts, and he had not agreed to them.
Ganapathi, now 22, on Monday pleaded guilty to two counts of sexual assault by penetration. Another four charges of using criminal force with intention to outrage modesty will be taken into consideration during sentencing.
In mitigation, Ganapathi's lawyer, Mr Louis Joseph of Regent Law, said his client had romantic feelings for the younger man.
He said: "He does not hide his sexuality, nor does he apologise for it".
The court has called for a report on the suitability of probation and Ganapathi will next appear in court on Nov 18.
On each of the charges proceeded with, he faces up to 10 years in jail, caning and a fine.
Get The New Paper for more stories.