I think Laura that the 'what does this mean' depends on a) whether dad is still on HT and b) if not, what his previous experience of HT side effects was.
If he is still on HT then this steady rise suggests that some of the cancer cells are learning how to feed without testosterone OR the hormones are not working as well as they used to. In either case, the onco might want to add another hormone to the mix.
If he is not currently on HT, then the rise might simply be because some of his healthy prostate cells are repairing and multiplying. Or it could be that a few cancer cells escaped the RT and are now making themselves known. If it is healthy cells then his PSA should settle again. If the rise continues, he will probably be advised to go back on the HT long-term. Different hospitals / oncos have very different views about when HT should be restarted though (some would do it at PSA2 while others would advise waiting until 5, 10 or even 20.
Whichever what, if dad is not regularly seeing his onco then now is a good time to ask the GP for a re-referral.