Yes, I still ride my bike regularly, and I rode it to most of the radiotherapy sessions at the Christie (I am an all weather biker, but thunderstorms are a bit much). Traffic and parking is so much easier on a motorbike.
Assuming he is having HDR brachy, it will be an overnight stay, with general anaesthetic, he won't get much sleep, as hospital wards are not good. (BTW food is excellent, if I get a recurrence of cancer, I'd be happy just to get another portion of Key Lime Pie).
So if you can give him a lift home after the brachy that would be good, other than that it is motorbike all the way through.
As others have said with a prostatectomy you would need a good few weeks for muscles to heal. Not so much for general riding, but for when the bike falls over at the super market carpark, and with great embarrassment you try and pick it up before everyone has noticed, and give yourself a hernia.