When he's alone with the doctor he tells him, "Doctor, I have a problem with my penis."
"That's OK," said the doctor. "Let me take a look."
The man got nervous and said, "Well, before I drop my pants, I need you to promise that you won't laugh, because my penis is rather small and it's a sensitive subject for me."
The doctor replied, "Look, son, don't you worry. We are professionals here. And, by the way, I have seen it all, believe me. I won't laugh."
So feeling more confident, the man dropped his pants and the doctor leaned over to look. He then started to laugh, and tried to contain himself but just started to laugh harder and harder. Finally he was able to control himself a bit, and while wiping the tears from his eyes, he asked the man, "Now what was your problem?"