He got letters from a number of "chemo sceptics" who suggested alternative therapies such as cayenne pepper and vodka, meditation and even oranges.
Downs had some witty responses to those but one suggestion would prove to be his lifesaver.
A reader told him about CAR T-cell therapy - a revolutionary new approach to cancer treatment which harnesses the body's immune system to fight cancer cells.
A clinical trial was to be held at Massachusetts General Hospital, a teaching hospital affiliated with Harvard University and they were willing to include Downs in the trial.
"I think they took pity on me because I was so far away but I also had a type of cancer that the CAR T-cell therapy targets," says Downs.
The estimated million dollar price tag was a problem and while friends and fans rallied to set up various fundraising ventures, Downs and his wife Katherine made plans to sell their house.
As it turned out the treatment was less expensive than anticipated and ended up costing around $350,000.
The couple and their three sons Jack 20, Joshua 17 and 13-year-old Toby flew to Boston where Downs' first appointment was booked for December 26, 2017.
"I called the kids Superman (Jack), Batman (Joshua) and Green Lantern (Toby).
"Partly because they were such amazing guys, helping us go through cancer.
"But also because as a joke, Toby changed the 'auto correct' on our phones so that when I wrote their names, it changed it to superhero names and even the teachers at school know what we mean when we say 'Batman will be late for school today'."
While his family returned to New Zealand for the beginning of the school term this year, Downs stayed on to recover from his treatment.
"It was a one-shot treatment but they had to genetically engineer my cells before injecting them back into me to fight cancer in my body."
Downs is now in remission and believes he had a "fairytale outcome" which should be available for others.
He has launched the Down with Cancer Campaign and hopes to raise $1 million to fund a clinical CAR T-cell therapy trial at New Zealand's Malaghan Institute in Wellington.
Malaghan director Professor Graham Le Gros says it will be the first such trial in New Zealand and stresses that the first trial, scheduled to start next year, will be a preliminary one.
"The treatment the Malaghan Institute is working on has been developed as a third generation CAR T-cell therapy, which is hoped to be more effective and easier to deliver than the treatments currently available elsewhere."
Le Gros emphasises that the treatment targets B-cell cancers, such as certain types of leukaemia, lymphoma and myelomas.
If the preliminary trial produces favourable results, a second trial will offer treatment to up to 13 people who have exhausted all other treatment options.
Because CAR T-cells are a relatively new type of treatment, they do not replace more proven therapies but will be used when people have not responded to the treatments conventionally offered.
Le Gros says Downs' campaign is a huge help to the institute and praises his magnanimous attitude.
"Rather than just being relieved at his own recovery, he wants to facilitate the opportunity for others."
Downs' parents, Ted and Maureen Downs of Whanganui, say they are extremely relieved that their son is in remission.
"David's gratitude knows no bounds," says Ted.
"He is addressing meetings throughout New Zealand and hopefully there will be one in Whanganui."
Downs describes himself as "a pretty decent public speaker" and bookings can be made through his agent.
To make a booking, donation or find out more about the Malaghan Institute visit malaghan.org.nz
