Peter Watts, 15, left a note for his parents at his home in Colwyn Bay on 19 January 1976, saying he was planning to help a school friend with homework.
He was found hours later collapsed in a Euston Road underpass and died in hospital.
The Metropolitan Police confirmed it was speaking to a potential new witness.
A post mortem examination gave the cause of death as head injuries consistent with a fall and there were no signs of a struggle.
The Metropolitan Police's special casework investigation team appealed for information as Peter's only surviving relative - his 57-year-old brother Mark - said this could be his last opportunity to find out what happened.
A man has come forward with information about the death of a Conwy county teenager 40 years after his death.