Neil Harwood claimed he could no longer recall crucial parts of the evidence he gave to police the day after the fatal collision in October 2016.
At the time he made a statement to police saying his brother David had told him and their father that he had been "punching in" a postcode on his satnav just before his BMW ploughed into the back of Dylan's bike as he rode home with a friend along a country lane late at night.
But when asked a number of questions about his evidence during the schoolboy's inquest at County Hall, he consistently claimed he could not recall what his brother had said.
County Coroner Dr James Adeley pressed Harwood after hearing the statement he made in 2016 had been used in evidence at a Crown Court hearing in March 2018 - a trial which eventually collapsed.
Finally an angry Dr Adeley urged lawyer David Woods, representing the driver David Harwood, to tell brother Neil "what my powers are in relation to a witness who won't answer questions."
The hearing was told that in the statement Neil Harwood gave just 24 hours after the tragedy he admitted he had spoken to his brother the morning after the collision and he had told him: "I'm screwed."
He told a detective: "David told me and Dad he was punching in (to) satnav and the next thing he knew was 'bang.'"
There were two mentions of the satnav claim in his statement, which he read through before signing.
But yesterday he could not remember having mentioned it and said his father could not recall the conversation either.
Dr Adelely told him that at the time he was interviewed by police and had been warned what would happen if he wilfully misrepresented something in his statement, he still signed it at the bottom of each page.
"Five years later you have made no attempts to correct that," he said.
Harwood claimed: "I think I got a bit muddled up about the satnav situation. I'm not sure about it."
The trial in 2018 collapsed part-way through and David Harwood was formally found not guilty of causing the death of Dylan by dangerous or careless driving on the direction of the judge.