Toby, aged five, has battled stage four Neuroblastoma since his fourth birthday, but was declared to be remission just months ago.
The progress came after his family - backed by Leeds United - launched a campaign that raised the £200,000 needed to pay for antibody treatment.
Toby’s mother, Stacey Worsley, said: “We’re absolutely devastated and numb. Toby has had to be in and out of hospital since around August 20 and it seemed to be that he was improving in the last couple of days.
“The doctors had planned to do the CT scan and I was convinced it was going to be clear.
“We thought he might just have a bug.
“He had an ultrasound recently to check he was still in remission and that was fine.”
Doctors hope to start a course of chemotherapy to try to shrink the tumour in the next two weeks, she said.
“The scan he has had doesn’t give the size of the tumour but he’s having an MRI scan this week and that should. The doctors think it’s bigger than a golf ball.
“We don’t think it’s spread anywhere else and so the doctors think there’s a better chance of reducing it in size. It may not clear but they’re saying they can try and shrink it down to basically nothing and it shouldn’t bother him.”
Toby seems “absolutely fine” in himself, Ms Worsley said. “You wouldn’t think anything was wrong with him.”
Support for the family has flooded in on social media, and Ms Worsley said: “We’re very grateful for all the love and prayers people are sending us on Twitter and Facebook.”