Hey & welcome.  I watched a couple of your son's videos.  He's doing well strumming chords in time & singing along.  Well done!

What to focus on next depends on his current skills & desired musical goals.  What style does he want to play that he can't yet?  What skills does he want to build or improve upon?  

Sometimes these are difficult questions for a student to answer until they see the next thing that motivates them.  It looks like your son knows basic open chords and can strum them in time on an acoustic.  If he wants to continue with acoustic, accompanying himself to sing songs, then the Acoustic course is probably the best option.

https://www.guitartricks.com/course/acoustic1

In that course he'll be able to continue expanding his knowledge & skills on acoustic guitar by learning about different rhythms, chord embellishments, fingerpicking, some basic theory & how to adapt all those new skills to different genres.  And if he likes it, he can continue that path with Acoustic level 2.

Hope that helps.  Please ask more questions if you have them.  Best of success to you & your son!