Location tracking

If his phone does not appear in Find My (assuming he set it up and authorized you to track him) there is no way you can track his phone. You can file a missing persons report with the police; they can request his approximate location from his cellular carrier.

You can use Find My on any Apple device, or by logging in to https://icloud.com/find using his Apple ID, or your Apple ID if he previously authorized you to track his phone.

Possibly they have stopped sharing their location with you. See the pic below, which may explain their action

You may again send a request to them to start sharing their location with you.

See the pic below which is self-explanatory.