Once a week medicine is injected into a port on his chest, sitting under the skin and leading directly to a vein. The other three times a week, medicine is injected by John's parents directly into his veins.
But finding the vein can be difficult. Mrs Davis says it's not uncommon for them to have to try more than once.
"John is quite happy to get the first needle in, but it's when we miss and have to say 'we need to do that again'.
"That's the hard bit about it, looking up at your 3-year-old and saying 'I'm so sorry, Mummy missed I have to try again'."
Earlier this year Mrs Davis learned of a machine, called Accuvein, which uses infrared technology to highlight where veins are so there's less chance of missing.
Mrs Davis pulls it off the shelf to show the Advocate reporter how it works.
With a 'dinosaur tooth' in hand, John pulls up his sleeve and his mother shines the light over his hand.
The machine is like an x-ray for veins; illuminated on top of the little boy's hand is a map of veins.
Priced at $7500, procuring the Accuvein seemed like a dream. But, Mrs Davis' friend, Simone Jaunay - or "the nicest person ever" - set up a fundraiser on the Givealittle website in late October.
By early November the $7000 goal had been reached - $2390 donated from the public and $4610 "selflessly" given by an anonymous donor.
"We can't ever repay that kindness but it's going to significantly make our lives easier," Mrs Davis said. "There's no way that we could have bought it without getting into debt. It's incredible."