It may not be easier than other proprietary stuff, but it is easy enough and probably the easiest you can get from the free software side. Just install Element

It may not be easier than other proprietary stuff, but it is easy enough and probably the easiest you can get from the free software side. Just install Element on iPhone or Android and make a matrix account from there, then add each other by the selected username. You will only have to help your grandpa when you start to add secondary devices that need to be authenticated the first time with a QR code (for decryption purposes). I have been using Element to text my wife for around 6 months (Android to iOS). I took the opportunity when she was getting notification issues with a proprietary app to convince her to use it. She loves it now. Clean interface, no ads, and voice call works just fine for us.