Lawyers representing Harry Dunn's alleged killer Anne Sacoolas have applied to dismiss a civil claim for damages filed against her in the US.
Mr Dunn, 19, died when his motorbike and a car driven by Mrs Sacoolas crashed in Northamptonshire.
She later claimed diplomatic immunity and returned to the US, which blocked a request for her extradition.
The prime minister has told the Dunn family's local MP the government is "committed" to supporting the case.
Radd Seiger, spokesman for the family, described the attempt to dismiss the civil claim as "pouring salt in the wounds" but said the parents "take comfort" from the prime minister's involvement in the case.
'Raised repeatedly with Trump'
Last month Mrs Sacoolas issued a statement saying she drove on the "wrong side of the road for 20 seconds" before the fatal crash outside RAF Croughton on 27 August 2019.
The American was charged with causing death by dangerous driving in December but an extradition request submitted by the Home Office was refused in January.
The US State Department has since said the decision to reject the request is "final".
Mr Dunn's family filed a civil claim at a court in the Eastern District of Virginia for damages against Mrs Sacoolas, last month.
In a letter sent to the Conservative MP for South Northamptonshire Andrea Leadsom, Boris Johnson said the government would "fund their accommodation for the family to attend court hearings".
He also offered to file an "amicus brief" in the case - meaning he would offer the court additional information it may wish to consider.
Mr Johnson said he "raised this case repeatedly with President Trump" and "regretted" there has been no change in the US position over extradition.
The Dunn family's lawyers have until 14 December to respond to Mrs Sacoolas's application.
Her legal representatives have been approached for comment.