If he hasn't already done so, he should definitely have his thyroid function checked as that can be a source of fatigue that will only get worse if it isn't caught and it isn't uncommon for the thyroid to quit working after RT. He should also be sure he's getting plenty of nutritious food and good hydration since lack of either of those things can contribute also to fatigue.

In my experience, when I went back to work full time, I had a much harder time getting the nutrition I needed--this was partly because I was still having major swallowing problems but even any isues with being able to eat normally, which many people still have at 5 months, can affect what you eat during work hours. Especially since for many the "norm" of eating during work hours is to grab something fast and eat it while you work and that is often not possible for people recovering from treatment for OC. So he may need to make really clear to his work colleagues what his needs are for time to be able to eat a nutritious lunch.

Your brother is a very lucky guy to have a sister like you supporting him!

Nelie