First of all, the standard caveat -- no two people are alike when it comes to treatment response & course. Having said that, given his problems to date, it would not be unreasonable to expect that things will worsen a bit over the next few weeks, then begin to turn around. Although Brian and Gary can probably provide a more cogent explanation, it is my understanding that the radiation is a cumulative effect, not the reaction to the immediate treatment. So, while many feel great for the first few weeks, the cumulative effect of the treatment starts impacting them after that. Under the same theory, the cumulative impact would build for awhile after treatment.
I was fortunate, in that my effects were relatively mild. I have frankly had more problems after treatment with the nerve impact of the neck dissection and fibrosis from the radiation, but I am not complaining.
Once the initial post-treatment period passes, things can start improving at a pretty good pace. I am just over 7 months out, and have most of my taste back, can do pretty much anything I want, and can eat almost anything, provided I have enough water with me.
My motto throughout treatment was "Prepare for the worst, rejoice at the best." While this worked well for me, it has its toll, too. Going through 8 - 9 weeks of treatment mentally preparing yourself each day for some new awful side effect wears on you. I had a real problem with depression in the early aftermath of treatment, and still battle it off and on.
It is something to watch out for. Also make sure that he gets plenty of rest. Getting over tired seems to make my depression worse.