The Tradition of Mother Son Dance
Why do I have to have a mother / son dance at the wedding? You don't. There is no hard rules about the matter. However, I am going to speak to you as a mother of a male child. Who is all grown up. Here is the reality...To the bride, while he is dating you he is private about his feelings. While you are planning the wedding he is worried about the reality of it all. He doesn't want you to dislike his mother, just like you don't want him to dislike yours. The "in-law" thing is difficult. Your brides parent get to choose what the grooms parent get to plan. The reality, if you are a single mom, what is more important to you than your child's happiness. This applies to both, mothers. Why not?
The tradition is there is no tradition, no etiquette other than the couples, first dancing together. It's called the first dance. They break and include the brides parents. They break and include the grooms parents. The best man and matron of honor are include then the wedding party joins in. At this point the parents are dancing together, if they are divorced and remarried with their spouce. The dance ends with all couple dancing with their spouce.
The point of this for the mom to feel part of his life. It helps them continue their relationship. For the relationship to continue. For communication to continue. Just because he is getting married doesn't mean he stops being her family.