Personally, I think XY is horrible for beginners and one of the reasons is because there are so many Pokemon to catch. It's overwhelming, and if you try to catch them all as you go through the story then beating the game will take forever. It also means that there is an insane amount of mons you have to learn the traits of just to clear the story efficiently. In this regard, I think a limited Pokemon pool of around 200 is better for less experienced players.
Also, the game's too fucking long. Both of my playthroughs were over 50 hours. For comparison, AS was about 20. And the reason XY took so long? Pacing. Horrible pacing. The game's way too drawn out. XY isn't drawn out because it has a lot of content, it's drawn out because the game's poorly designed. Several stretches of the game, including very early on between gyms 1 and 2, are just dreadful. ORAS, however, was fun for me the entire way through. It was a nice, brisk pacing.
It's also outdated, so if he likes the battle system and decides to get into battling others then he's going to have a hard time finding good opponents because they'll have moved onto ORAS. Also, the Friend Safari is useless now because nobody does safari adds anymore. The DexNav is infinitely more useful.
Speaking of battling, hatching is a hassle in XY compared to ORAS, EV Training is a pain in XY compared to ORAS, and leveling is a horror in XY compared to ORAS. Also, stuff besides battling? XY don't got any aside from some shit you'll get done in like 2 to 3 hours. ORAS, however, has contests, secret bases, and a fuckton more legendaries to catch.
Speaking of which, having all those legendaries to catch is very important. Non-legendaries are typically very easy to trade for because you can just breed more, you know? Not the case with legends because you can't just breed more legends. So in terms of completing the dex, he'll have a significantly easier time if he starts with ORAS instead of XY because the hardest part of completing the dex is legendaries. Similarly, if he decides to get into battling he'll need ORAS's legendaries.
Things like trainer customization and promo videos aren't important. They don't have a real impact on gameplay. For a newbie's first Pokemon game you want a fun, briskly paced adventure that doesn't have an overwhelming Pokemon selection and I think ORAS fits that better than XY.