Stephen Sutton, from Burntwood in Staffordshire, was diagnosed with incurable colorectal cancer at the age of 15.
Last year he was told it was inoperable, prompting him to write a bucket list of 46 things he wanted to achieve.
One of his goals is to raise £1m for Teenage Cancer Trust, and he has been using the internet to help him achieve this.
His JustGiving page has since gone viral, helping him raise £250,000 in a day.
So far, more than £820,000 has been donated.
Celebrities have been retweeting it on Twitter, and the funds keep on coming.
Piers Morgan tweeted: “What a remarkable, inspiring, courageous young man … please help him achieve his total.”
Stephen posted on his Facebook page on Monday about his deteriorating condition, saying:
“Currently I am in a very vulnerable position and do not know what will happen next.
“My right lung has collapsed and I am very weak (I’m finding it hard to type this message)
“I will fight as hard as I can, but if the worst happens I just want to say thank you all for sharing the journey with me- it’s been wonderful.”