I would ask why the change in protocol. Explain that you are not resistant, but maybe the oncologist has a good reason to suspect residual cancer? I know a PET gives false positives too close to finishing treatment, maybe a CT is less so but still I cannot imagine with all that he's gone through that it would come back clear. Healing, inflammation, infection - all of that show on a CT - and likely all of that would be present post treatment. Plus rads continues working for up to two weeks post treatment.
After my treatment was done the oncologist signed off on me, and left me in the hands of my rads dr. and Surgical oncologist/ENT.

;o) best of luck.