Robert was aged 15 when he was reported missing from his home in Resolven, Neath.
Despite extensive enquiries and numerous appeals for information over the past eight years, police have been unable to locate him.
Following a review of outstanding missing persons in the force area, a team of detectives are now conducting enquiries into the case.
As part of the investigation, police have released an age progression photograph to depict what Robert Williams would look like today.
Detective Inspector Mark Lewis, of the South Wales Police Major Crime Investigation Team, said: “All outstanding missing person cases remain open and are revisited periodically in case new evidence comes to light.
“I have a team of officers investigating the disappearance of Robert Williams who went missing from his home in Resolven almost eight years ago. As part of that process we will assess, re-visit and evaluate all information gathered up until now as well as identifying any potential new lines of enquiry. I will remain determined to seek an outcome for Robert ’s mother, family and friends.
“We are appealing to anyone who may have information about his disappearance to come forward and contact the police.
“His mother, Cheryl Davies, is still deeply anxious to find her son. I would urge anyone with information to contact us.”
Contact the South Wales Police Incident Room on 01639 889103 or Crimestoppers anonymously on 0800 555 111.