A few things from my experience.
You have a tendency to become less continent during the treatment and for the month afterwards. I switched to wearing disposable pants half way through. I never did have an accident, but knowing it didn't matter if I did was good security.
Most people get a change in bowel habits. Usually, this is looser stools, wind, and mucus (mucus farts as my oncologist puts it), going more often, and also losing the ability to sense if a fart is safe or not. For the looser stools, it may be necessary to switch to a low or very low fibre diet, but don't change diet until/unless it becomes necessary. I had to stop eating fruit and veg and anything with brown flour, brown rice, or seeds. There are a few exceptions, peeled apples, pears, bananas, potatoes are low fibre and OK. This slowly improved from a month after the radiotherapy, and was just about back to normal 5 months later, except for broccoli which took a year before I could eat it again. Another effect is a false sense you need to do a #2 when there's actually nothing there to do.
In a small number of cases, the opposite happens and people get constipation, in which case you need to eat more fibre.
Pelvic floor exercises are really important, and I think I only managed to stay continent because I'd been doing them. He can start doing those now.
One other thing, at some point, you may lose the ability to do just #1's as distinct from #2's, i.e. you may find you managed to s*** yourself while standing to pee. This is another temporary effect which recovers a month or so after treatment, but if this happens, it's best to always sit down to pee until it recovers.