They had a Jumpstart product, where you could buy two half-decks and mix them together. But it looks like the last printing of that was 2022.
If you don't mind spending the time, you could just craft a starter deck on paper or find a list online, and then order the individual cards from a website. As long as you avoid the must-have cards for competitive players, it'll be cheap.
See also: https://mtg.fandom.com/wiki/Core_set
The game is complex, but a good intellectual challenge for a bright 6yo reader.
Also, FWIW, 5th edition also didn't have a set symbol, but the "Visions" set was released at around the same time and had a "V" symbol... (I only realized how confusing it was after 27 years lol...
Alternatively, Star Wars Unlimited just released their first set. The starter set has two balanced decks (Vader vs Luke) with some staple cards that can be used in a bunch of different decks with boosters. Gameplay is Magic-esque, but smoothed out (no land issues since any card can be turned into a resource, no instants/stack shenanigans, and actions pass back and forth so constant engagement).
There's a free to play version at https://www.forcetable.net/swu using the starter decks.
Here is a small blogpost by WotC: https://magic.wizards.com/en/news/announcements/revitalizing...
The basic rules around commander aren't particularly additionally taxing beyond that. (This is your special card that you can play every time. When it dies it goes back to its special spot and it gets more expensive each time you cast it). What I like about the commander format (in the few pre-constructed decks I've bought over the last few years) - they can really lean into a theme/strategy more - and the commander really guides you in how to play this deck.
Start with those arena kits - they are cheap and handle the rules. But if your son sees a commander deck that resonates with him (in theme or mechanics) -- I'd recommend giving it a shot.
I was assuming I would go in and buy a couple of starter decks from the most recent numbered "edition" to start teaching him, but it appears that they don't do this anymore, and everything I saw was focused around the "Commander" format, which has even more mechanics and rules. The game was already complex enough that I was worried about teaching it to someone new (Plus, the commander mechanic didn't appeal to me, personally, anyway).
Is there no longer the concept of a "simple base set" with just the basic mechanics? Or is everything just theme-heavy sets now?