With a new set just becoming legal for sanctioned play this week, Standard is actually uncharted l water even for those of us who have been playing competitively for quite some time. There are a few decks that were known to be good because they are essentially converted from the Expanded format, which would be the following:
M-Manectric-EX / Regiice AOS, and perhaps other Manectric varients
Seismtiad-EX/ Giratina-EX
Vileplume AOS
Vespiqueeen AOS variants, with and without Raichu XY
Dark variants (Yveltal-EX / Zoroark BKT / other attackers)
Bronzong variants
Fighting (Lucario-EX/ Hawlucha FFI) / Crobat
The thing is with the format being so young there is a lot of opportunity for rogue decks to creep up, and especially the first week of City Championships, you can expect to see some unconventional plays while the metagame is being established. Depending on your and your son's willingness to practice, some of these decks take more skill to play effectively, while others have more straightforward strategy. You can easily master any deck with enough practive, but learning the ins and outs of some decks and how to manage resources doesn't happen after just a few games. Of the decks I mentioned, M-Manectric-EX and Fighting / Bats are more forgiving to mistakes, while Vespiqueen and other Double Colorless Energy dependent decks require more practice and resource management to master. I'm not sure what your budget is, but some of these cards can get expensive. As bbninjas said, Shaymin-EX is particualrly pricey, and several staples can be found in the Mewtwo vs Darkrai Battle Arena decks, so that's a good place to start. I'm probably leaving a few important things out, but here's a list of staple cards you should be looking for and their approximate values (remember, some of these are in those Battle Arena decks). This is on a per-deck basis, so if you want to run multiple decks (and I assume you want a deck for and a deck for your son) you will need to get at least two sets of what I'm listing.
Shaymin-EX (2-4 copies, depending on deck, $30+ each)
Professor Sycamore (4 copies, $1.50-$2 each)
Lysandre (2 copies, $1-$2 each)
VS Seeker (4 copies, $4-$5 each)
Judge (2-3 copies, $1 each)
Ultra Ball (4 copies, $2 each)
Muscle Band (2-3 copies, $3-$4 each)
Battle Compressor (2-4 copies, depending on deck, not needed in every deck, $3-$4 each)
Trainer's Mail (4 copies, not needed in every deck, $1.50 each)
Ace Trainer (1 Copy, $1 each)
AZ (1-2 copies, 4 for Vileplume, $2 each)
Xerosic (1-2 copies, $1.50 each)
Brigette (1 copy, $1 each)
Double Colorless Energy (4 copies, only used in certain decks, $2 each)
Hex Maniac (1-2 copies, $1 each)
Super Scoop Up (4 copies, only used in certain decks, $1 each)
Each deck also requires specific Trainer cards for support that might be unique to that deck. If you look around online you can find basic lists for the decks I mentioned above.