How to track a stolen iPhone?

In addition to the above: If your iPhone or iPad was stolen - Apple Support

You may get a call or text from “Apple” or the ”police” reporting having found your iPhone, and requesting you verify your identity with your Apple Account credentials. Providing those credentials would be Bad, as that’s the thief calling or texting.

Find your lost iPhone or iPad - Apple Support

If you are going to use your iPhone to try to track your boyfriend's phone......you will need to know his Apple ID and Password to log on to Find My.

Tell your boyfriend to log into his iCloud account at https://icloud.com/find