"We believe the victim lost balance and fell off the building," the source said.
The man fell on to the podium on the fourth floor of the building. He was certified dead at the scene by paramedics.
A police spokesman said initial investigations found nothing suspicious.
It is understood Kong had been working as a freelance trainer in a fitness centre in the same building for less than two months.
According to his Facebook page, Kong came second in the Olympia Amateur Hong Kong Men's Physique 2016 event and was sixth in this year's Hong Kong Federation of Body Building and Fitness Hong Kong Men's Physique competition.
He previously had four years of experience at one of the city's top personal training gyms.
- South China Morning Post