can I enter a cell phone number for an apple phone and see it displayed on GPS

See the following from If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Find My [device] is the only way that you can track or locate a lost or missing device. If Find My [device] isn't enabled on your device before it goes missing, there's no other Apple service that can find, track, or flag your device for you.

If he has an idea where it was misplaced (e.g., in a house) try calling it from another phone and listen for the ring.

No but he can use any internet browser and log into www.icloud.com/find

Also see: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

My g=son misplaced his phone And I would like to know how to locate it without Find My turned on. The phone number is tied to my ATT account.