One of the great things about the Young Eagle / Sporty's ground school is it is the same ground school they offer to all potential pilots, it qualifies you to take the FAA written test (instead of needing an instructor's endorsement), and they track the Young Eagle's process through the course so that when he/she completes the first half, EAA automatically sends out a certificate for a free 1 hour flying lesson at the school of your choice (EAA pays the flight school for that lesson).
The course does not have to be taken right away. If your son is younger (let's say 8 for example), he can wait until he is older to take the course. After all, if he takes the FAA written test and passes it, it is only good for 2 years before it has to be retaken, so it's better to wait until he is within 2 years of getting his license before taking the test (must be 17 years old to get a license). They keep a record of the kids, so it he will be able to safely wait and take it later.
This is not "ground school" per se, but still a good resource on systems, handling, performance, aerodynamics, etc.: https://www.av8n.com/how/ Although I haven't read every word, I believe it's a generally reliable source.
I took the Dauntless PGL prep 4 years ago, and passed the knowledge test with a 97%. This disappointed me. Ended up dropping out of soaring before my check ride.
Three years later, committed to doing it all over (Knowledge Test had expired) and finishing. Using Dauntless, and really studying the things that had given me difficulty, especially VORs, received a 100% score on the PGL Knowledge Test (written).
If there are gliders near by, your son can solo at 14.