This needs to be followed up with your son's specialist nurse or medical team. He may have lymphoedema which can caused by lymph glands not working properly either due to the disease, you don't say where the cancer was at diagnosis, or caused by damage to the abdominal lymph glands from the treatments. There can be residual internal swelling from RT for instance . The lymph glands are unable to perform their normal function of draining bodily fluids hence the swelling. It needs proper diagnosis and treatment if it is lymphoedema. Some Macmillan centres have specialist nurses who advise and treat, also there is a charity, the Lymphoedema Support Network I think it's called, who have an excellent help line and will advise on treatment.
My partner had genital lymphoedema, though he had an indwelling catheter. It had to be removed as the swelling was causing discomfort around the tip of the penis where the catheter came out. Your son's problem may improve after the RT but the last thing he needs is increased problems urinating. I may be over reacting here but please advise him to get it checked out. My heart goes out to him, very early to be diagnosed with advanced PCa but we do have men in their 40's and 50's in a similar predicament. You will get a lot of help and support from the site, we have many relatives and partners who post here, Regards,