Cisplatin made me sick too. It was precisely 6 days after the infusion and only lasted half an hour. I never threw up and I am pretty sure that the nausea was mostly due to me not taking the anti nausea meds after a couple of days...
Once I was off cisplatin there were no more nausea problems at all and I stopped taking any anti nausea drugs. So (at least for me) radiation did not do it.

My memory is not crystal clear for this period either but there may also be a psychological component to this in my case.
********If he is in pain the he needs adequate pain management !! People with oral cancer are on fentanyl patches and or Morphine/Oxycodone etc. I would venture the guess that excessive pain does not help memory either.
Advil? Are you serious? What does his RO/MO say? Ne needs something better than that. Also you might want to consider viscous lidocaine** for short term relief (for eating/speaking). This allowed my to titrate the use of the strong stuff.

The treatment is miserable and will get worse, but there is an end to this; do not loose sight of the long term goals. Before you know it you will have the luxury of worrying about all kinds of silly things again.

M

** I am beginning to sound like I am on a crusade. I am not. It helped me and I am surprised it is not more used.