How can i use my find my iphone app to locate my husbands lost iphone

You can use Safari (or any internet browser) and log into www.icloud.com/find with his Apple ID

Also see the following from Use the Find My app to locate a lost or stolen device - Apple Support

If you've lost or misplaced a device, here are some of the things you can do. Just use the Find My app on another iPhone, iPad, iPod touch, or Mac signed in with the same Apple ID. You can also use the Find My app on a device owned by someone in your Family Sharing group, or sign in to iCloud.com/find to locate your device using a browser. 

Additional information in If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support