From 50 mcg to 100 mcg is quite a leap. Typically Fentanyl is increased in 25mcg increments. My bloodwork was very low in both WBC and RBC (so what was in there anyway?!?!) but I can't remember being delusional (although my wife may differ with my opinion on that). Fentanyl does have a long half-life and also takes a full 24 hours to come up to speed.

I hate to bring this up but has he had an MRI of his brain to rule out it being tumor related?

This close to the end of radiation is when all sorts of "interesting" things can happen. Malnutrition, disfunctional immune system, thrush, dehydration, severe constipation, etc. It may well be a multidimensional problem. Certainly a 103 degree temperature, in it's own right, can make one delusional.