Hi Diane,
About the scans, I suppose if you fussed and hollered you might be able to change the doctor's mind, if that is what your father really wants, if it will help ease his mind. Maybe it's that the doctor wants to spare unnecessary worry.
I think your assumption that the treatments are/were palliative is correct. Palliative treatments aren't supposed to be worse than the disease. So, it makes sense to skip the chemo if your father feels worse afterwards. And I can see the logic of not doing treatments and/or scans unless you need to run after something acute-something causing pain, and treat with radiation and pain meds.
Perhaps something less toxic is being tried in a clinical trial setting, you can always ask if there are options if your father is up to the idea.
Skipping the last of 6 cycles might or might not make a big difference, it's kind of hard to say. Did he show good response to the other cycles? If he is too weak, blood counts too low or just very sick from the treatments, it might not be worth it, could leave him open to infection and most of all, wreck quality of life.
Quality of life is paramount at this point, and scans are not what defines this.
These are tough questions and very tough decisions that ultimately only your father can make. He is lucky to have a loving daughter out here looking for some answers on his behalf.
I wish you both all the best,
Wendy