What Causes Severe Pain During Bowel Movement With Diarrhea?

My husband earlier today went to the toilet and passed a very painful 'number 2'. When it started to come out it was very painful and ended with diahorrea. Since then, my husband has had 3 occurances of 'severe stabbing pain' about an inch to the right of the top of his 'crack'. These are not prolonged pains, they last for around 20 seconds each time and he describes the pain as having a knife being slammed into his buttock. He is 40 yrs old, 5' 10" and has arthritis in every joint in his right leg. All of these posterior pains have happened whilst sitting down for about half hour beforehand.