My husband has lost his phone. How can I add him to my Find my iphone so we can find it?

If you aren't using Family Sharing, you can use the following from Locate a lost or stolen device - Apple Support

(or you can use any computer internet browser and log into www.icloud.com/find)

You can help a friend find their device if it's missing and has Find My [device] turned on:

1. Open the Find My app.
2. Choose the Me tab.
3. Select Help a Friend.
4. Have your friend sign in to iCloud with the Apple ID they're signed in with on their missing device.