How do I add my son's iPhone to this it is lost trying to find it

It is too late to enable the Find My option on the phone that is lost, if that is what you are asking.

If Find My was enabled on the phone.....before.....it was lost, then you need the Apple ID and Password of the person whose phone is lost in order to try to track the phone.

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

Keep in mind that the phone will not show up on the Find My map if the battery has run down or the phone does not have a good connection to the Internet where it is currently located.