Oscar had been given the all-clear from the condition in April but writing in a blog on the family's website on Thursday night, Oscar's mum Leona revealed they had found out last week that the cancer had returned.
"We've had some terrible news - Oscar's cancer has returned, the disease is extensive, and it's spreading rapidly," she wrote.
Our little man is in excruciating pain, not eating or drinking, and finding it very difficult to walk. The priority right now is to relieve his pain with regular and increasing morphine, and to decide on and begin a treatment plan ASAP."
Leona said her son faces an "incredibly tough battle" and that she and her husband Stephen were "shocked, scared and overwhelmed" at the news.
Hundreds of thousands of pounds have been donated over the past 18 months to help pay for treatment for Oscar to give him a chance of survival.
Instead of receiving the life saving treatment, Oscar was left fighting for his life in America when a heart complaint was diagnosed.
He fought back from the brink twice and his courage throughout his tough battle inspired a huge following of 30,000 people on Twitter.
Little Oscar was honoured in June at the Sunday Life Spirit of Northern Ireland awards where he received a standing ovation as he was presented with the main award on the night.