The family of the 29-year-old intern who was slapped by his supervisor, identified only as Alan, is demanding $100,000 in compensation for lost wages over the years.
They had previously visited the office to confront the supervisor, but had refused to accept his apology to settle the matter.
According to a report in Lianhe Wanbao, the intern had joined Encore eServices's Jurong East office three years ago as an undergraduate. His salary was just $500 a month and not given any benefits or leave days.
When he had graduated from university, the intern continued to work for the company for the same pay, still without a contract or any benefits.
His family said that he often returned home late and also turned up at work even when he was ill.
According to the intern, his supervisor had physically and verbally abused him from time to time. However, he said that the worst incident was the one secretly recorded and shared online.
STOMPers Beejay and Justin had contributed the video to STOMP on Saturday (May 18), and expressed shock at the man's treatment of the intern.
When asked, the intern told a Wanbao reporter that he did not leave the job as he liked it very much and learned many new things. He also said that he felt like he could not leave because there were always projects that he had to finish.
Despite that, the intern had tried to resign once in 2012 but was told not to leave as the office was shorthanded.
When he asked for a pay raise, he was told by his supervisor that his performance was not up to scratch and that he lacked experience.
Meanwhile, two other female former interns have also stepped forward to tell of how they "worked in fear" at Encore eServices because of their boss' explosive temper.
One of the women - who asked to be identified as Miss Liang, 23 - told Chinese language daily Shin Min that she worked at the company during an internship arranged by her university in 2011.