How can I trace my husbands lost phone on my phone?

You would have to sign out of Find My iPhone app, then log in with your husband's Apple ID and password to track his device. See the instructions here. If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

It's real easy IF you guys use Family Sharing. Log into iCloud and your husband's phone (as well as yours) will be shown on Find my iPhone.