Oscar Saxelby-Lee had a transplant in May after a search for a donor saw thousands across the UK sign-up to a register in a bid to help.
Following the procedure, Oscar was briefly cancer-free, but now the cancer has returned.
Oscar has T-cell acute lymphoblastic leukaemia, which is difficult to treat and means he has been in and out of hospital for the past nine months.
The only real option left for Oscar and his family now is in an overseas trial called CAR-T, which would mean travelling to Singapore.
With the cost of treatment, travel and insurance to cover, his parents Olivia Saxelby and Jamie Lee, from Worcester, must now raise the huge sum in order to give their son a chance.
Olivia Saxelby said that the highs and lows of 2019 had been difficult to take, but they still had hope.
"Every time we've had good news, it's been shocking, horrific news to follow," she said.
"But that's the nature of this disease - it's not just us (going through it)."
"We have to accept this is Oscar's journey but all we can do is try and make it better by curing him and getting him home where he belongs."
Following his relapse, Oscar is back on a ward specially for stem cell patients at Birmingham Children's Hospital, where he is undergoing treatment to try and keep the cancer at bay.
She added that Oscar's diagnosis in December 2018 was a "massive blow".
"Then we found out he needed a stem cell transplant."
Olivia Saxelby added Oscar sometimes "gets really upset with life" and it was heart-breaking for him not to be able to do what other little boys his age take for granted.
However, she said her son was also "incredible" and a fighter.
"His little dimples keep us going and he's a cheeky chappy, always has been, always will be," she said.
"As much as he is kind of isolated from life and normality, he's still a little boy, he's still great fun and still loves to have fun and enjoy everything around him."
"We have now been put in a predicament where we have a price to pay for our child's life," she added.
Update: Oscar's mum Olivia Saxelby said she and his dad Jamie Lee has been left "absolutely overwhelmed" by the generosity of friends, family and strangers, as donations for their appeal have poured in over the past week.As of September 19, over £140,000 has been raised of the £500,000 target.
Olivia said: "The support has been incredible. We have been bombarded with texts, Facebook messages, social media, all in support of Oscar."
"I just cannot express how much it means to us and Oscar."
"Oscar knows money is coming in to his appeal, he knows people out there are thinking about him," she added.
The family are also encouraging people, community groups and organisations to hold fundraising events, like dress-down work days, or by wearing something red - Oscar's favourite colour.
The fundraising drive is all leading up to "Oscar Day" on October 11, when the family hope to be in reach of their target.