Before, mum Charlotte Caldwell, from County Tyrone, had seen the oil confiscated by United Kingdom customs at Heathrow after bringing a new batch from Canada, i reported yesterday.
His condition led to Home Secretary Sajid Javid later approving the return of some of the cannabis oil, after doctors made clear it was a medical emergency.
She added: "The Home Office, myself and my team have been working extremely hard throughout the night to make this happen, which is truly fantastic, but there can only be one conclusion here: that my handsome sweet little boy, who has a life-threatening form of epilepsy and one seizure can kill him, he needs his medicine back today".
Ms Caldwell's success in obtaining the cannabis oil, even for short time, was hailed as "triumphant" by Barbara Zieniewicz, co-founder of campaign group Families4Access, who travelled with Ms Caldwell to Canada.
British Home Secretary Sajid Javid on Saturday granted 12-year-old Billy Caldwell the use of cannabis oil, as the epileptic boy was facing a "medical emergency".
The boy's family said he was taken to hospital when his seizures "intensified" in recent days.
Speaking to BBC Breakfast, Ms Caldwell said: "This experience, that myself and my little boy have endured in this last week, I do not want, and I will not stand by and let, any other family in our country endure".
"He has 20 days worth of anti-epileptic seizure drugs".
However, she said his body and his mind have been "completely broken" after the ordeal.
Charlotte who has fought tirelessly to secure medication for Billy for a number of years has said that doctors back home and in Canada have said that his case is now life threatening.
Cannabis oil is banned in Britain.
"This is about getting the medication that these children throughout the United Kingdom desperately need to control their epilepsy".
Sinn Féin MP Órfhlaith Begley welcomed confirmation that Billy would now get his treatment. She said that the Home Office would be held accountable if he died.
Last year 12-year-old Billy, from Co Tyrone, became the first person in the United Kingdom to be given a medical prescription for cannabis as treatment for epilepsy.
Charlotte subsequently met with Home Office Minister Nick Hurd who told her the medicine would not be returned.
His increasingly concerned mother initially took him to St Mary's Hospital in London on Thursday night, where doctors were horrified that he was being deprived of the treatment, their statement added.
Ms Caldwell said she thinks no other family should have to travel "half-way round the world" in order to seek medication.