Please call his specialist nurse and/or ask to speak to his consultant again. This is an important decision.
Radical prostatectomy is done with the aim of curing the cancer.
Radiotherapy here would also be aimed at cure - not just shrinking and controlling the cancer. If he is unsure about treatment I would recommend that he sees on oncologist (expert in radiotherapy) too.
His life expectancy would not be 10-15 years from either surgery or radiotherapy.
There used to be a school of thought that you shouldn't offer curative treatment for prostate cancer unless the patient was likely to live for another 10 + years, as if not they would probably die with, instead of die from their prostate cancer.
For example if you had an 85 year old with heart problems, intensive treatment of the prostate cancer is unlikely to increase their lifespan and so you would probably 'watch and wait' or give hormones alone. This spares the patient from the side effects of surgery or radiotherapy.
I wonder if that is where you have got the 10-15 year figure from?
Surgery and radiotherapy are both effective treatments for prostate cancer, but have different side effects.
When you speak to the consultant ask about complication rates including risk of impotence, incontinence and bowel issues, and the likelihood of needing further treatment. The fact that he has also been given the option of 'active surveillance ' means that he must have a fairly low risk cancer. Active surveillance is different to the watchful waiting offered to the v elderly or unfit. He would be monitored more closely with plan to intervene with surgery or radiotherapy if signs of cancer growing.
Good luck.