The mother, who remains anonymous, shared the image on the Love What Matters Facebook page on 20 February with a moving description.
She said she posted it for anyone else going through the same battle and to show others how cancer can “destroy” a person.
“For anyone going through this horrible disease, it is about to get real, real hard and real quick,” the mum wrote.
“The picture I’m posting is from this morning, and before you scream and cry ‘Why would she post a picture of him in a pull up?’. Well... because life is not always politically correct and pretty, it is real.”
The mum explained the photo was taken after carrying her son, Drake, to the bathroom. She said he was wearing a nappy as he is no longer able to control when he needs the toilet.
“This is skin and bones, because I have to beg him to eat one green bean for supper, or drink a cup of water throughout the day,” she wrote.
“This is having your son sleep with you at night because he is afraid of something happening and being alone, and by something I mean dying.
“This is having middle-of-the-night conversations with a 10-year-old, who is asking if he dies will he go to heaven and will he see his dad there and be able to talk and play with him.”
The mum explained Drake often throws up his medication because his stomach is empty. She said he has to take 44 chemotherapy tablets each week.
“This is him telling me he is scared and thinks he won’t see his 11th birthday,” she added. “This is him and me, telling him that I will continue to fight for him when he can’t.
“From the moment I found out I was pregnant till future forever, he has been my reason for life. He is my smile, my love, my heartbeat. He is also my tears, my heartache, my frowns. He is my life.”
The mother’s story was liked nearly 250,000 times within 15 hours of being posted and had more than 42,000 shares.
Many people commenting said they wanted to gift the mother and son with letters of love, encouragement and support - but didn’t know how to contact her.
The boy’s aunt commented, writing: “I am this little boy’s aunt Ka. We will be setting up a PO box soon. Right now we are just trying to get through the day. We never excepted this to spread as much as it has. Thank you for your care and support.”
An admin on the Love What Matters page wrote: “A mailbox for letters of love is currently being established, we will update here when it’s ready, thank you to everyone for the beautiful display of support.”