The terminally-ill Sunderland fan, who suffers from a rare cancer called neuroblastoma, received the "devastating" results on Thursday, according to the Bradley Lowery's Fight Facebook page.
Bradley, from Blackhall, near Hartlepool, has moved people around the world with his battle against the disease and has even struck up a friendship with departing Sunderland footballer Jermain Defoe.
The youngster was sent for a chest X-ray after recording low oxygen levels following his latest radiotherapy treatment this week.
A statement on Facebook said: "The results were devastating, all of Bradley's tumours have grown really fast.
"The reason his oxygen levels are low is because there isn't much room in his lungs now.
"I was told Bradley has now only got weeks to live because of how rapid the cancer is spreading.
"Yes I knew this was coming but I thought it was going to be months not weeks.
"My heart is broken in two, it is too soon for this to happen, it should never be happening to start with.
"My baby has had to put up with so much over the past four years and now he has to suffer a painful death.
"How is that fair??? Why should any parent have to be put through this heart ache???"
It comes just over a week after Bradley's family said scans had revealed new tumours, and the six-year-old was now receiving palliative care at home.
Last month Bradley was joined by striker Defoe and Sunderland keeper Vito Mannone for his birthday party.
The celebrations also included fire-eaters, a funfair, marquee, jugglers and stilt-walkers. Bradley also walked out with Defoe for his England comeback match at Wembley.