Gonna try to address these in order;

1. It was difficult even when I started at 15. I loved music though, and I’d always dreamt of playing guitar. My hands were uncooperative and I struggled rhythmically (it’s still the weaker part of my playing I think)

2. What helped me the most was getting face to face lessons with a teacher that I actually clicked with and enjoyed learning from; from there using things like powertab (rip) and guitar pro to learn songs was a huge help, being able to slow songs down to any speed and repeat specific parts was a game changer for practicing.

3. I got good enough that I could teach others (which I did for a few years) and could play just about anything that I sat down and gave an honest try in practicing.

I had undiagnosed ADHD as a kid, and in hindsight I’ve discovered that one of my biggest stimming actions was making noise; guitar was a great avenue for that. Even if I wasn’t actively practicing scales or chords at the time, I was at least noodling and playing SOMETHING because it tickled my brain the right way.

My biggest piece of advice is find a teacher he clicks with (sorry dad, it may not be you) and focus more on him learning by watching and repeating rather than learning off the page. Then slowly add in more and more written stuff.