I want to see my son's iphone in Find My iPhone

My son got a new iPhone. I want to be able to see his phone on my iCloud account where I can see all of our family Apple devices. He has Find My iPhone turned on in his phone under his iCloud Account. I signed out as him on his phone and signed in as me. But his phone will still not show up in the Find My iPhone on the computer with the rest of our devices.

I tried to setup Family Sharing from my Windows computer on the Find My iPhone account but don't see an option for that.

I tried to setup Family Sharing from my iPad but it won't let me proceed because it is making me turn on Share Purchases and add a credit card. I don't want to share purchases, just be able to find his phone on my Find My iPhone along with all of our other Apple devices.

What do I try next to get this to work?