What about safe mode? Only basic functions work. 

 I’ll dig more into “safe mode” on Android, thank you!! 

 It’s basically a debug mode. Only the most vital components are active. This mode is used to minimize the variables of an issue you may be having.  

This is probably the scorched earth approach, there may be simpler ways. 

 1. Buy pixel
2. Install grapheneOS
3. Do not install google playstore, it's optional
4. Create a non-admin profile for him
5. Done, nothing else can be installed by him

Regarding phone calls, I don't know if that is possible but it would be an interesting feature to implement (a whitelist for calls instead of a blacklist), you can suggest that to the devs, they are very responsive. 

 If the phone reboots, you'll need to take a drive over or whatever to unlock the Owner profile for him... 

 True, didn't think about that. Maybe that forces regular visits, which might be a good thing 😄 

 This is so helpful and I’ll check it out, thank you so much! 

 Maybe go to settings>digital well being and parental controls. 

 i need to check 4 same/maybe dumb phone/ already w/pixel & graphene os/still working it out 4 my older friend