I am sorry for your father's
constipation and
anal pain. Constipation happens commonly in elderly people, but it is not caused by aging itself. Some things that can make constipation happen more often are lack of mobility and exercise (which sounds likely with your father if he has a urinary catheter), medications, and certain medical/health problems.
Sometimes when there is stool
impaction from constipation, liquid stool passes out of the
rectum around the blockage, and I am wondering if the loose stools you are noting are due to impaction.
If he is having extreme pain, I urge you to take him to the doctor, clinic, or hospital for an examination urgently. There may be internal problems that are not visible when looking at his anus. Also ask the doctor to review your father's medical history and current medications to determine if any diagnoses that your father has, or medications he is on, may increase the chances of constipation. In addition, if there is bowel impaction, which is not apparent from the outside, the doctor can physically help relieve that.
If there are no anatomical problems or medications that need changing, you may want to give your father a
stool softener such as Docusate.
I am also providing a link to an article on causes and management of constipation. http://www.aafp.org/afp/2005/1201/p2277.html