The CT scan will show the extent and position of the cancer. As you've discovered, the purpose of the CT scan is to plan RT. If Danny has RT this would mean that surgery would become impossible. So, the oncologist has ruled out the option of referral to a surgeon. Question 1: why hasn't surgery been discussed with Danny. Q2 Another is the type of RT. From what you say it seems that External Beam RT (EBRT) is planned. There are variants of RT - Intensity Modulated (IMRT) with image guidance is now widely available and is definitely preferable to the older conformal beam RT. Q3 Another question is whether brachytherapy is a possibility. Q4 Also ask whether hormone therapy is planned to continue after RT. This decision depends on the nature and extent of the disease.
I know someone who has recently gone on AS with a low risk cancer: Gleason 6 and early staging, T1c. He has been told that PSA will be monitored for some months, than another MRI followed by some months later another biopsy. Dependent on the results of those tests - spread over about 18 months - treatment would be discussed.
Most importantly, doctors recommend treatment, but patients decide. Remember that "Knowledge Empowers". Without knowledge Danny may take the wrong decision.