The week before his wedding, the groom gets a swift kick in the nuts causing him to see his girlfriend’s baby as a grown man in a onesie who guides him and tests him on his readiness for marriage and fatherhood.

fiance's not fiances (A bit of a difference)

This could be done using the "A Christmas Carol" template but Instead of a 'ghost' guiding him he has his fiances "Grown child" showing him the consequences of his life.

Great comments! The addition of the descriptor "man-child" would enhance this logline. The Antagonist is the infant, as perceived as a man, since he is challenging the Protagonist to his suitability to be a husband and father, the end goal.
Thanks for the constructive comments.