I use Parsec. I used Chrome Remote Desktop in the past, which was good; I've used Splashtop which was amazing too.
Parsec is good enough that I can remote play games on my computer from work without much of a delay. It's designed for gaming, so I rarely if ever have any issues with video corruption or not displaying a piece of software.
You can just make a remote-in account, set up 2FA (it's TOTP based, not SMS crap) and log in both places -- his machine will just show up for you to connect to whenever he wants help.
As far as I'm aware, it supports Windows and Mac hosting.
If you already have a pi over there, a PiKVM would be a good choice too - in case you needed to reboot/access bios settings, etc too.