I've had this a few times and put it down to the HT. I'm on Prostap too and have been for a year and 4 months. After thinking it's the cancer spreading and having my good wife try to put my mind at rest of course :-)
I've noticed that when I lower my excercise rate as you need to when having treatment (or a few lazy days) muscle wastage is so much faster now than when not on HT. With all the muscles in your back it's hard to keep them all active and he's been through a lot.
I've found falling asleep on the sofa at a funny angle, sleeping in bed funny or this week building a snowman with my son caused some real unusual back ache and weakness which can last for days.
Hope it's cleared up for him
Matt