I've taught quite a few six year olds, and I'd say the keys are:
1. Use the Pokémon they want to use.
2. Make a deck that doesn't require abilities or similar, at first
3. Use supporters and items that are fairly straightforward
4. Don't use too many cards that require shuffling
For DW Necrozma, I'd try to keep it simple; focus on Psychic Pokémon. Necrozma (UNM) is one that the kids enjoy, for example. Then just see if he has any other favorite Pokémon, and maybe included those - if he likes MewTwo you can use the UNB MewTwo (it has an ability, but it's not critical). Say 12 pokémon in total, or a bit more, kids tend to do better with fewer mulligans as those take forever.
Then add something like:
10 Supporters:
Cynthia x2
Hau x2
Professor Kukui x2
Lillie x2
Judge x2
24 Items:
Switch x4
Escape Rope x2
Escape Board x2
Acro Bike x4
Rescue Stretcher x2
Nest Ball x4
Mysterious Treasure x2
Choice Band x2
Vitality Band x2
14 Psychic Energy
That balances easy to use cards, some but not too many strategic decisions (some are really obvious, like "Choice or vitality band" which hopefully he can figure out; some are a little more complex, like Acro Bike choices), not too much shuffling balanced with consistency (some Pokémon search, and Cynthia/Judge, but not any else) since shuffling is the bane of the six year old; some "fun-killer avoiders" (all that switch/etc. stuff in particular, worst thing is having a Pokémon stuck in active as a kid), and some "fun" (Judge, in particular, is just fun to play on your opponent). Most of the supporters give more cards (Yes, Hau is in there, you can switch for 2 more Kukui if it hurts your brain, but at this age 3 more cards is really nice).
If he really struggles with shuffling dump the Cynthia and Judge for Hau and Kukui, and dump some of the Pokémon search (but not all) for more Pokémon.
That's just a skeleton, do what you want around that for sure - just focus on simple to pilot, but not zero choices, and no complex mechanics - especially no abilities. It doesn't have to be a world beater - maybe make it around the level of a "good" theme deck, or even a "double" theme deck (where you take 2 theme decks and use the cards to make one good deck), and then play with theme decks against him. Over time he'll get better and you can give him better cards!