Potsie,
Being sick does not give him a license to treat you badly. "Honey I am sorry you feel bad. This does not give you the ok to be nasty to me. I am just trying to help. It is ok to tell me to back off, but you need to do it in a manner that is respectful. (i.e. "Mom I appreciate your caring, but please back off." and then you need to actually do it) Especially if you want to be treated with respect." Bear in mind that is very general advice and is suitable for people over the age of 10.
But there is a caveat that if things like this keep happening please speak with a therapist. There is chronic illness therapy, and want to encourage you to think about it even if your son does not want to because it is a heavy load to be a caregiver. Please also bear in mind that it might take you several different therapists to find one whom you feel comfortable with. I am concerned about you.
I wanted to point you toward this thread in prostate cancer, because I want you to know you are not alone and it might help you to read it.
https://www.healingwell.com/community/default.aspx?f=35&m=1720907
Hugs,
Navy
P.S. If I could take back all the nasty, unsupportive actions behaviors that my Mom put up with I would.