Private GPS tracking

maybe try these:
https://famisafe.wondershare.com/mobile-tracker/child-phone-tracker-apps.html

 

The downside of installing tracking without them knowing (which is really "spyware", however it is dressed-up) is that if they do discover it they are likely to view this as a much greater violation than your looking at their Google location history. Just saying that there is a balance of risks here.

(Of course if they are up to no good and aren't terminally stupid they will turn off Google location tracking - and if they know how Google work they'll turn off backdoors like "app and web activity history", which provides a way of Google getting a location history when you think you've turned it off! But Google design their systems to not draw user's attention to how much logging of activity they do if you just accept the defaults, so maybe that won't occur to them).

 

Nothing good ever comes from spying on a family member.

 

Do you have access to his Google Account and his phone? If not, I don't think you can do anything.

 

There's hardware for that.

 

Tiny.