Can you provide some more info about the drugs he is taking. Reading the bords here, hallucination (generally) seems to be tied to painkilllers, and changing them helped. Radiation (IMRT?), should not fry your brain, they avoid irradiating brain tissue. What chemo did he receive?
Radiation and chemo (especially for 8 month) really knocks you on your ass and it takes a long time to recover form this. This is not something that once the TX is done you pick up and go again...
From personal experience, after radiation (2 weeks) even a walk from garage to lab was a "death" march because I was that weak.
You also need to consider the emotional impact that this may have (depression etc). Many patients struggle with this in different forms.
I would definitely talk to his MO about this.

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