Lawyers for the family of Francis Rowntree, who was struck by a projectile fired by a Royal Anglian Regiment soldier in west Belfast in 1972, have established the onlooker's name but are still trying to trace his whereabouts, a preliminary inquest hearing was told.
Previous proceedings had heard that one of the soldiers involved in the incident was in very poor health and would not be able to attend the new inquest.
A lawyer for the Ministry of Defence on Monday confirmed to coroner Jim Kitson that the MoD considered it "inconceivable" that he could give evidence.
Eleven-year-old Francis was hit by the rubber bullet as he played with friends at the Divis Flats complex close to the Falls Road in April 1972.
He died four days later from injuries including a fractured skull.
Controversy surrounds the shooting, with disputed claims on whether the young boy was fired on directly, or hit by a ricochet, and if the bullet had been doctored to make it potentially cause more injury.
The Rowntree family barrister, Fiona Doherty, said efforts were continuing to find the witness, who was 14-years-old at the time.
"His name has been traced but not where that person is," she said.
"Himself and his family have moved out of the jurisdiction."
It is thought the man, who would now be in his mid-50s, may be living in Wales.
It is understood he was not interviewed by the authorities in the wake of the incident, but did give an anonymised account to anti-rubber bullet campaigners.
Ms Doherty suggested to Mr Kitson that his office may have to make efforts to contact the man.
Mr Kitson confirmed that he would be willing to seek the man, with the police, to obtain a statement.
"He would be in a position to provide extremely cogent evidence in this case," he said.
Earlier, barrister for the MoD Ken Boyd gave the court the latest update on the health of the ailing soldier.
He said military officials did not consider him fit to give evidence due to heart problems.
"The MoD consider that it's inconceivable," he said.
Mr Kitson said the court would need to obtain medical reports to confirm the man's condition.
Mr Boyd said the commanding officer inside the military vehicle from where the shot was fired - the only other soldier interviewed about the incident - was willing to give evidence.
"But he does say that he doesn't recall the incident," he added.
The schoolboy's death was among 14 controversial killings during the Troubles for which Attorney General John Larkin had ordered new inquests.
Proceedings were due to begin last November - but were dramatically halted when Northern Ireland's senior coroner John Leckey questioned whether the Attorney General had exceeded his powers.
The Rowntree family and a number of others launched a legal challenge against the suspension and it was subsequently lifted in February.
Mr Kitson scheduled another preliminary hearing for mid-September.
After the hearing, solicitor for the Rowntree family Padraig Ó Muirigh urged the witness to come forward.