It's confusing isn't it! :)
The Galore Park book is Attic Greek. It is a nice intro to Ancient Greek and might be a good taster kind of course to see if Greek is really your son's thing.
The most popular (and, IMO, best) Attic Greek textbook is Athenaze. It's commonly used in university Greek classes, but two of my kids started it in 7th grade and had no troubles (we went fairly slowly, of course). It took DD 15 three years to complete Athenaze I&II and DD 13 is on track to finish in a similar amount of time.
My one hesitation with mixing the Galore Park book and the Athenaze book is that they use a different order for declensions. :glare: Athenaze follows the nominative, genative, dative, accusative, vocative pattern, whereas GP goes nominative, vocative, accusative, genative, dative. It's not really a big deal, so long as you stick with one format, but it can be confusing at first.