Tracking an iPhone

Dear Apple Community,

I am really hoping you are able to help me please.

I have given my son (13) my old Iphone 3GS.

My question is that i would like to keep an eye on him and track his iPhone 3GS so that i know where he is.

His phone is updated to IOS 6.1.6 and it won't update anymore.

I have an iPhone 6.

How can i track his phone as find my phone needs to be IOS 7 or more and i find that other tracking apps also have to be IOS 7 or 8 etc?

Is there any way i can put IOS 7 or 8 onto his phone?

By putting a better version of IOS on to his phone it will also mean that he can play certain games (like Pokemon Go)

Any help would be grateful received.

Ralph

PS: I am not tech minded so please keep fairly simple.