The UK and US have agreed to end the “anomaly” that allowed the woman suspected of causing Harry Dunn’s death to claim diplomatic immunity.
A court heard last month that the “secret agreement” permitted Anne Sacoolas to return to her home country after a road crash which killed the 19-year-old in August last year outside RAF Croughton in Northamptonshire.
Despite the document now being amended, Harry’s alleged killer still remains in the US.
His mother Charlotte Charles said the announcement was a “huge step forward” – adding that one of the family’s aims was for this to “never happen to another family again”.
But she added their campaign would continue for Ms Sacoolas to be brought back to the UK.
Foreign Secretary Dominic Raab said the changes mean a similar tragedy “could not happen again”.
He added: “I know these changes won’t bring Harry back, and I appreciate the pain and suffering the family are still going through.
“But I hope this may bring some small measure of comfort to them, because I know they want to prevent any other family going through the same ordeal they have.”
It follows a visit by the US Secretary of State Mike Pompeo to the UK this week.
Ms Charles appealed to the government at the weekend to make her son the “top priority” during the visit and said she was tired of promises.
“We’ve been assured by Dominic Raab’s team…that Harry is number one on the agenda,” she said.
“Well prove that to us then. Tell us what they’re going to do. Words are not enough anymore.
“There are plenty of things [Mr Raab] could do, we need proof of what he’s going to do. He looks defeatist, he looks like a little lost child in an adult’s world. Stand up for us, we are UK citizens.”
A previous request for the extradition of Anne Sacoolas was rejected by the US in January.