Does he have an electric guitar or an acoustic? If it's electric, my kids (and husband) have learned a TON from Rocksmith (it's not cheap but is a one-time purchase, and it comes with tons of songs). It has a lot of games and lessons to teach the skills and chords. It uses a real guitar, hooked to the computer with a cable (that comes with the program, I believe), and the computer tells you exactly what you've hit and missed, and it adapts as your kid progresses in ability. You may need to keep an eye on what songs your 12yo plays if you are particular; there are a couple of them that are not clean enough IMO for kids, but there are many classic, decent songs.
I got these this book and its sequel for my husband to use a couple of years ago, and he has worked through them and then used them to teach the kids some more skills: https://www.amazon.c...e?ie=UTF8&psc=1