Hi!
This may not be quite what you are expecting, but the single best thing you can do is arrange transport to/ from the hospital: navigating NHS car parks at busy times is universally a nightmare and a major source of stress.
Beyond that, given that your step dad seems to be active and in good general health, he should not find the experience too traumatic: the majority of people on here report more issues with the Hormone treatment (if that is running in parallel) than with the RT itself.
The machine is noisy, but it's kind of fascinating in its own way
As he gets into the final sets of treatment, and for a week or two after, encourage him to stay close to a loo; but otherwise the more active he can remain, the better. Oh, and its never too late to start pelvic floor exercises.