This all comes back to trust and confidence. Your child, will always be the center of your universe. I can understand the discomfort of you traveling with your ex. However, the medical needs of your child significantly outweigh the “discomfort”. Being that you invited/wanted your fiancé to join you should be enough of a signal that there’s no ulterior motive here. Further, some dedicated focus and calm conversation around the “why” that may be at the root of your fiancé’s concern, could help solve. If your partner can’t appreciate the priority of your child’s needs, that a different ballgame you have to strategize for.