I can't imagine what you're going through. I have a 4 year old grandson and he is precious. As others have said soups and bone broths would be good. They are nutritious and easy to digest.
This might sound weird but I would give him breast milk! I am a huge supporter of extended breastfeeding in children anyway but there have been studies that suggest it may actually help fight cancer and improve quality of life. If you are not lactating there are ways that you can make yourself start lactating again. You may not feel comfortable breastfeeding him at this age, but you can always pump. It will provide him with much needed nutrition and immunities.
Use of Human Milk as Cancer Therapy: Breast Milk Helps Reduce Cancer-Related Symptoms | Suite101.com
Few highlights about cancer.
I had no cancer and in family had only one, but I am fascinated about cancer as it seems it is not a disease or sickness.
Cancer might be natural human healing ability turned off.
It is greatly about what we eat and how we live.
As many have came to conclusion, before age of agriculture, cancer was mostly non existent, that says something.
I would suggest you to check Apricot kernels as one interesting way of aiding little one(there is no overdose records, except only one and it is shady).
Vitamin C, high dosage, either IV or Liposomal Encapsulated Vitamin C(best choice).
Can't stress enough about eating the old ways.
My sincerest condolences to you. I believe you are absolutely right with everything you believe and if it was my son I wouldn't let anyone with junk food touch him with a ten food pole.
I second the vitamin C but I would give it to him in the way of food - Oranges, red peppers, strawberries..
I wish I could offer more.
I just wanted to say that I'm holding you and your son in my thoughts. I have two boys, 5 and 1. Just know that this stranger is thinking of you and sending you light and love.
I agree with many of the previous posters. I just started giving my boys bone broth made from organic chickens. I understand that it's an important part of the GAPS diet for healing the gut. I'm sure the information that everyone else has provided is sufficient.
Please let us know how it is going for you and your son, if you can.