Frank Rowntree was fatally injured by security forces during rioting near Divis Flats in west Belfast in 1972. The 11-year-old died days later in hospital.
The HET found he was a completely innocent bystander, and was taking no part in riots.
A damning report into the inquiry team was publicised by Her Majesty's Inspectorate of Constabulary on Wednesday, finding that investigations into Troubles killings involving British soldiers should also have been more rigorous.
Frank's brother Jim said the HET investigation was supposed to bring his family closure - but instead they are reliving the schoolboy's death.
He told UTV: "The memories are still quite fresh of Frank, and the state that he was in then, lying in intensive care at the RVH.
They say time is a great healer, but we are not getting any time at all because they keep changing the goalposts
"All those things come back to haunt you. You think of them all and it must be terrible for my mother, having to relive this again in her own head."
In its report, the HET said it traced the soldier responsible and found that he was in poor health, suffering from dementia, and could not clearly recollect what happened.
The soldier wasn't tracked down - in fact UTV understands he called the unit.
But it has now emerged the HET told the family in a letter it was all too plain to see an attempt to interview him under caution would have been ludicrous.
In reality they never saw him at all.
The Rowntree family trusted that the HET probe would bring the truth about Frank's death, but his brother said they were left shocked by the latest findings.
"My family put their faith in them after what they told us at the first one or two meetings," continued Jim Rowntree.
"Then everyone went quiet and then all of a sudden this has come out.
"I think they have failed Frank and the whole family - where at the start I thought we were going to get somewhere with them, for a stupid phone call to wreck the whole case, it's stupid, why was it even put down in black and white?
"What else are they going to come with next?"
Commissioner for Victims and Survivors, Kathryn Stone, has also voiced her concerns following the publication of the scathing HMIC report.
The consideration of victims and survivors and their families should always be central to the work of the HET
She continued: "This report will affect those who have already been through the process of having their cases reviewed by the HET, those who are currently going through the process, and those who are still waiting.
"It is imperative that effective communication channels are established by the Chief Constable to deal with any concerns that victims and survivors may want to raise after today's report."
Relatives for Justice Director Mark Thompson called for the HET to be scrapped.
"The Minister for Justice David Ford and the Policing Board must move to immediately scrap the HET," said Mr Thompson.
"The Minister, with the British Secretary of State Teresa Villiers, must now stop prevaricating and pave the way for creation and establishment of an independent, Article 2 compliant mechanism to properly investigate all state killings."
A statement from Wave Trauma Centre said: "There have been failings identified in investigating cases involving use of force by the state, which are particularly serious, however many of the faults relate to how the HET deals with all cases more generally.
"It is not a case of raising the standards around one group of investigations, but rather need for a fundamental change overall."
PSNI Chief Constable Matt Baggott apologised over the HET's policies and said all military cases will be re-examined, giving an assurance "that this has ended".