At the moment, he's in a double whammy of causes of tiredness.
HT causes tiredness, and the workaround for that is exercise. Indeed, exercise makes a big positive difference to many of the side effects. Many local prostate cancer support groups run weekly gym exercise classes cheaply or for free, and you should see if you can hook up with that.
The RT also causes tiredness because it will be lowering the red blood cell count (I lost 8% in 23 RT sessions, it will be more for someone on a full 6 week treatment). This will also make him feel tired, adding on top of the HT tiredness. This red blood cell count will take about a month to recover after the RT finishes, and until then, it's probably better to avoid any strenuous exercise, but some leisurely walking should be in order.