they have to tell you that because it is a small one. Just like a commercial has to tell you all the possible side effects of a medication. If even one person had a side effect it has to be listed as one. Same with clomid and twins. I know more people who have used it and not had twins than have.
If he is that freaked out then I highly suggest you talk to your doc about Femara (aka Letrozole). The possibility of twins (while it is still there) is only 1-3% in couples with no twins in the family. It also has less side effects on you, and has a short half life in the body so it doesn't affect your uterine lining like clomid can. clomid is known for causing the lining of your uterus to thin. Which, of course, is counter productive to having a little bean attach properly.