My son lost his phone today, we are on the same apple account, I have find my phone app. How do I put in his number to find his phone?

Charlie-brown- wrote:

How do I use my lost my phone app to find his?

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

His number or cell service account is not involved. If you use the same Apple ID (which should not be done), his device should appear directly in the Find My App on your device.

If you are not using the same Apple ID, then you need to log in to iCloud.com with his Apple ID and use the Find My app there.