I seem to have a very similar problem...similar, but different. I had a total proctocolectomy with end ileostomy on June 28, 2010. My problem was ulcerative proctitis, with which I suffered for twelve years. I was also 63 at the time of my operation. My surgeon left the anus intact, but I was informed that the rectum and colon were gone. I had my surgery at Pennsylvania Hospital in Philadelphia by the head of colorectal surgery. I had no idea he was going to leave the anus. Two weeks post op, I developed a hematoma inside the anus. The surgeon was able to drain it during an office visit
He explained that there are more complications when the anus is removed (his opinion) and, like in your father's case, if there is mucus, it is able to drain out instead of settling inside.
My problem is that I believe I have a rectovaginal fistula which NO ONE can find!!! From time to time, I have some bleeding from my vagina (I am more than 20 yrs post menopausal) and for the last year or so, I have quite a bit of mucus which drains out of the vagina, not the anus. My surgeon retired in 2013 and I am seeing another colorectal surgeon here in south Jersey. The disease is inside my anus and the rectal cuff. He did extensive biopsies which showed NO dysplasia, no cancer. I was told to use Anucort suppositories, which is a steroid. Well, I have such little room in there that I cannot insert an entire suppository and it caused a lot of discomfort. He then told me to use Proctofoam, which when inserted causes burning inside my vagina!!! They cannot find a fistula, but because of my symptoms, they agree there must be one. Since there is no feces passing through, they are not concerned. For the past month or so, I have seen quite a bit of mucus and must wear a panty liner. I also have been to my gyn doctor since my operation and had three transvaginal ultrasounds, PAP tests, MRI's and all negative. So, for the moment, I just have to live with this because they said to undergo an operation for removal of the anus would be too much of an operation for me and the outcome may be worse than what I have now!
It's early on since your dad's operation and I think he is just going through the healing process. However, ask his surgeon if he should undergo yearly biopsies of the anus. My surgeon didn't tell me that, but the doctors I see now instructed me to do so.