He shold discuss this with his doctors.  You won't get qualified medical
advice here.

Having said that, let me tell you what I know.  Frequency of urination
is probably related to irritation of the bladder and urethra from the
radiation.  His doctors should certainly know about that if it is taking
place.  But one needs a certain amount of fluid in one's system to avoid
dehydration, which can have serious medical consequences.  My guess, and
it is just that, is that he should continue to drink what he normally
does and just put up with the frequent urination during the radiation
therapy, if it is indeed unavoidable.  Of course that may limit how far
he can walk.  He can deal with that by taking many short walks or by
wearing a pad like Depends.