The 63-year-old is married with a daughter.
The incident took place in January last year, and Chia is now on standing trial for the alleged sexual assault.
The Sec 1 boy was sitting on the stairs at a block of flats in Tampines at about 9.30am when he Chua made friends with him.
He had run away from home and spent the night before at Changi Airport.
Chua asked the boy if he wanted to walk to the Tampines Eco Green park with him and he agreed.
He was allegedly relieving himself in a cubicle at the toilet when Chua barged in and performed oral sex on him.
The boy allegedly pushed Chua away and ran off, then took a taxi to his biological father’s home.
He went to KK Women’s and Children’s Hospital on his own in the evening, saying he felt unwell.
Meanwhile, the boy’s mother had filed a missing persons report at about 1.35pm.
She was reunited with the boy when the hospital called her to inform her of her son’s whereabouts.
Chua was subsequently arrested.
DNA was found on the boy’s shorts and the door handle of the toilet, while 2 palm prints lifted from the door handle also matched Chua’s left palm.
If convicted of sexual assault by penetration, Chua can be sentenced to up to 20 years jail.
But, he will be spared the rotan as is above 50 years old.