Brave Billy, who is from Castlederg and suffers from epilepsy, requires medicinal cannabis oil to help prevent seizures.
And his mother Charlotte Caldwell credits the treatment with saving his life.
Home Office officials have previously warned her GP to stop prescribing the drug to the youngster.
On Monday, Charlotte and Billy completed the trip back from Canada where he was assessed by doctors and provided with a prescription of medicinal cannabis.
Medication was confiscated when the mother and child arrived at Heathrow, as it is not certified for use in the UK.
“I’m just going to turn around and go get some more; and keep doing so until the UK authorities see sense.
“I take the view that I’d rather have my son illegally alive than legally dead.
“This is the scenario that the phrase ‘no brainer’ was invented for,” said Charlotte Caldwell.
Dr Dan Poulter, MP for Central Suffolk and North Ipswich, who has been supporting the Caldwells, said:
“The current law is ridiculous; there is growing evidence that cannabis products used medically can be helpful in treating a number of conditions, but yet is still seen through the prism of illegality here in the UK.
“It is simply inhumane that Billy’s medication, which is legal in many other countries across the world, has been confiscated.
“This is bound to have a devastating effect on Billy and his family and I will continue to do all that I can to help bring about this much needed change in our law.”
The Home Office has yet to respond to the seizure.