Find my iPhone

When I log into my son's icloud account to use the "find my iPhone" feature to locate my son's iphone, it sends a code to his phone that I have to enter on my end to access his icloud account to be able to use the "find my iphone" feature. This defeats the purpose because if the phone is lost, we can't get the code to see where it is on the find my iphone map. How do we use this feature properly?