I was diagnosed in June last year with PC (PSA 19 Gleason 4+3) and went on HT pills immediately then HT injections for a year. Also had Radiotherapy daily for a month in November as part of a PIVOTALboost trial that I signed up for. The diagnosis was actually a spin-off from being diagnosed with Myasthenia Gravis (MG) at the same time.
The treatment for the PC was parallel with MG treatment but meant I was getting i.a. steroids too.
All of this resulted in feeling absolute rubbish for a year with most of the classic side-effects, but especially tiredness, back ache and ballooning out due to the steroids (with resultant muscle wastage).
I finished the HT injections in June and am gradually feeling better but still get the odd back ache and the tiredness. PSA now down to <0.1. However...I am back in the garden and I am determined that the tiredness is now due to exercise.
I'm not sure if the above helps but I would encourage your father to try to exercise and recover the muscle wastage if he can. It was, for me, a very slow process and I understand that I am one of the lucky ones in that my side-effects are actually disappearing quicker than the average.
As everyone says, no two people are the same but I should hope that with your support and encouragement and your father's determination he should start to feel better soon.