The attack happened at Frederick Street around 6pm on Sunday.
However, police have only just released details of the incident.
The victim, described as “a young teenager”, was walking with another teenage boy, when he was sexually assaulted by an unknown man.
The attacker was described as being about 30 to 40 years of age, of average build, and about 5ft 8ins tall.
He was wearing a football top, black tracksuit bottoms and a beanie hat and had a scar on the left side of his face.
Chief Inspector Norman Haslett said: “I would appeal to anyone who may have been in the area at the time of this incident, especially the driver of a silver car who passed by, to contact police on 101.”