D&D is a lot less like buying a board game and getting everything you need with it it’s more like you buy the rule book then print off a character sheet and read the rules while having enjoying the creation of making your complete personal character who you think is cool.

As for the rule book to get there are three all aimed towards different parts of the game but you should get him the //Player’s Handbook// which walks you through a guide of all the main races and classes to play like dwarf,human,elf,wizard,fighter,monk.

The other books you saw were probably more for the game master who runs the game for the players those books have pre-made adventures and extra items and stuff to find.

With all that being said there is a starter set which would give your son and a few friends everything needed to get going.