Sam Tai, 17, has been diagnosed with global amnesia, a condition so severe he can’t remember how to find his way home from the shops.
Six weeks after the injury, he wakes every morning and “doesn’t know anything”.
But his proud mum Jane has revealed that even though he can’t remember where he lives, the keen karate kid can recall an entire sequence of martial arts moves lasting a few minutes.
The teenager’s plight is similar to Drew Barrymore’s character in the hit Hollywood rom-com 50 First Dates.
Doctors have told his family he should get his memory back – but there is no guarantee when that will happen.
Sam, a keen karate student from Fulford, North Yorks., suffered his memory loss on March 27 after being carried off during a junior rugby union game.
His mum Jane said: “He put in some big tackles playing rugby. After the second one, he was a little bit unsteady on his feet.
“He passed his pitch side concussion test and seemed fine, but then later that night he lost all of his memory.
“On the Monday he had a scan, but it looked fine. I took him back the following Monday to see the GP.
“She spoke to a neurologist and he was taken for an MRI straight away.
“That didn’t show anything either. He is due to see the neurologist again this Friday.”
She added: “He has a few random memories that pop up, but he can’t really remember anything from birth.
“Sometimes he remembers people he has been in constant contact with like his girlfriend Elly, but even then it’s very patchy.
“He can’t even go to the shops because he doesn’t know how to get home.”
Jane said Sam’s friends have been quick to help him, adding: “When he first wakes up in the morning, he really doesn’t know anything. Social media has been brilliant for him.
“All his friends have been messaging him. He can look back through his history and see who they are and what he likes.
“He didn’t know what music he liked, so it helped him there.
“By halfway through the day, he knows a lot more than he did when he started. The cycle then starts again the next day.
“He can’t really go to college at the moment. It’s all quite overwhelming for him.
“He goes once a week for a couple of hours just to sit in his class, but he can’t really remember his lessons.”
Jane said: “The doctors have just said we need to wait, which is frustrating.
“They are hoping that his memory will come back, but there’s no guarantee.
“I seem to remember his consultant saying that he hadn’t come across this before, certainly not to this extent.”
She said Sam’s ability to remember a karate routine has given them hope.
She added: “Sam is a black belt in karate. His instructor, Dave Cartawick, came up to visit him last Thursday to see if he could help.
“They were just messing around, but Sam ended up doing one of the katas, which has 64 moves. It was amazing.
“I put a video of it on Facebook and it has already had 50,000 hits.
“It has sparked a lot of interest. People seem to be really interested about how he manages his life.”
Luke Griggs, from brain injury charity Headway, said: “Memory loss following brain injury is a complex area.
“Amnesia can affect people in many different ways although severe cases such as this are rare without a prolonged loss of consciousness.
“Retrograde amnesia refers to the loss of memory for a period prior to the injury.
“Typically, this gap in memory will range from a few minutes to a few months, however each brain injury is unique and more extreme memory loss can occur.”
Headway, which provides support and information to families affected by brain injury, has this week launched a new campaign to educate grassroots sportsmen and women about concussion.
Luke said: “There is no way to predict how long it will take and to what extent the old memories will return and it must be a very difficult time for this young man and his family.
“It sadly goes to show that even a seemingly minor head injury can have a major impact, and the only positive is that he was removed from the rugby match before sustaining an even more serious secondary injury.
“It is vital that everyone who plays sport treats concussion with respect and takes an ‘if in doubt, sit it out!’ approach.”