3D Platformers on Xbox seems to be somewhat of a niche genre. It has plenty of them, but it's not as if they're exclusives, though I do reccommend the N64
Banjo duology. You can get them separately, or buy
Rare Replay that has 28 other games, along with
Banjo-Kazooie and
Tooie;
Conker's Bad Fur Day being another notable (if a bit more mature and satirical) example. Since the compilation has 30 games on it, and was $30 when it came out — thus being cheaper as a result, I don't think you can go wrong with buying that instead. Not every game is a gem, but you at least get some fantastic games as a result, some of the best in the medium, I think.
I should mentioned that there's a spiritual successor to
Banjo-Kazooie titled
Yooka-Laylee, though I still recommend playing the
Banjo games first.
As
@Flicky mentioned,
Super Lucky's Tale and
New Super Lucky's Tale are both good starting points. They're not too daunting in terms of difficulty and they're fine for what they are. I spent a good amount of time collecting as many achievements as I could for both games, and I even managed to 100% at least one of them. Your nephew might have a better time with
NSLT. The game was retooled to function more similarly to a typically 3D game with a 360° camera, and Lucky feels more natural to control. The levels are quite different from the ones in
SLT, along with other noteworthy differences, so it's more than just an enhanced version of its predecessor.
