Pain in the glandular urethra is often "referred" pain, often from the bladder. Men who have urinary tract infections will often feel the pain there when their bladder is irritated. Sometimes you tend to feel the pain where the nerve ends when the actual problem is somewhere else along the run on the nerve (in this case the pudendal nerve).
Lots of guys here complain of that pain although it sounds like your dad's is unusually bad. My theory is that in a lot of cases it is caused by irritation from the stitches that were used to stitch the urethra to the bladder neck. Those stitches will be attacked by his body as he heals and absorbed but that process can take months and months and in the meanwhile they can be irritating.
His problem could also be some sort of entrapment of the pudendal nerve in the area of his surgery.
The good news is that the pain usually goes away eventually. While guys are waiting for it to go away they will try all sorts of remedies and the remedy they are using when the pain goes away will be the one that they think worked.
Not much advice here. Talk to his doctor about
the possibilities that it might be "referred" pain. That'll at least give you some different things to try while he waits to get better.
Here are a few links you might check out. None of them apply perfectly but they will give you ideas of what might be going on...prostatecancerinfolink.net/2013/12/30/peripheral-pain-after-treatment-for-prostate-cancer/www.spuninfo.org/index_files/WhatIsPN.htmmalepelvicfloor.com/urinary.html