When My Son, Age 17, Had A Bowel Movement About

When my son, age 17, had a bowel movement about a week ago, it really hurt, and his butt has really hurt since. I was thinking that he had an anal fissure. However, he said that it doesn't really hurt when he has had bowel movements since. However, it really hurts when he walks, sits, runs, etc. When I asked him where it hurts, he said that it hurts between his anus and scrotum. We've been putting antibiotic ointment in the area where it hurts along with desitin. It doesn't seem to be working. What could this be? Should I take him to the Dr.?