Cash Hyde, known as Cashy, was a perfectly healthy baby when he was born in June 2008 but became sick shortly before his second birthday.
At first he was misdiagnosed with glandular fever before his parents Mike and Kalli, from Missoula in Montana, were given the devastating news he had a serious brain tumour.
The little boy had to have arduous chemotherapy treatment to reduce the growth, which had drastic side effects including seizures and a blood infection.
His distraught parents were repeatedly told he was likely to succumb to the illness because the condition was so bad.
After one bout of high-dose chemotherapy, Cash was so weak he could not lift his head and was too sick to eat any solid food for 40 days.
It was at this point that Mr Hyde decided to take action and go down the route of medical marijuana to try to help his young son.