Bradley Lowery, from Blackhall, County Durham, has touched the world of football through his battle to overcome his condition, striking up a moving friendship with England international Jermain Defoe.
The youngster has a neuroblastoma, a solid tumour which makes up 8% of the total number of children's cancer in the UK but whose cause remains unknown.
A post on his campaign Facebook page, apparently written by his mother Gemma, said Bradley was due to be a mascot for Sunderland's match against Middlesbrough on Wednesday evening but was not well enough to attend.
It read: "Bradley is not good, he has been in horrendous pain with his leg since yesterday to the point he couldn't move in bed last night.
"We have had him to the hospital today and they have given him an X-ray but it didn't show anything.
"The doctors' gut feeling is that it is his cancer progressing but we need to wait and watch for a few days to see what happens.
"I am hoping and praying that it is nothing serious and he starts to improve as I'm not ready for this to happen yet."
Bradley got a rousing reception from England fans as he walked out as a mascot for last month's World Cup qualifier against Lithuania at Wembley and an online fundraising campaign for treatment has raised nearly Â£67,000.