The mother-son dance is the quiet moment of the wedding. It is the woman who raised him releasing him into his own family. A custom song for it lets her see what she made and lets him say thank you without having to be perfect at it.
What this kind of song holds
These songs hold the years of his childhood that she carried alone or together. They hold the lessons that landed. They hold the moments she let him become his own man even when it cost her. The best ones don't over-explain the love — they trust the audience to feel it.
What to put in the intake
Tell us a moment from his childhood she still tells. Tell us what she always said. Tell us what he carries from her. Tell us the song she sang to him when he was small. We weave those into a song specific to her — not to "every mom."