How do I locate spouse's misplaced iPhone 6S?
You cannot simply add a device to Find My iPhone - though Family Sharing will enable that automatically. Think of find my device like data backups, you need to set them up before you lose the item in question. 

Unless you are already set up for Family Sharing, which would allow you to see the location of all the family's devices, then from an iPhone using Find My iPhone, or a web browser using https://www.icloud.com/#find , sign into his Apple ID.

Otherwise, check with your cellular carrier or check wireless networks to see if the phone is joined to any known networks.