I suspect you would create an account for yourself, then use family/parent settings to restrict what your son's account has access to. But I've never used parental controls, so I'm not entirely sure how that whole process goes.
https://beta.support.xbox.com/help/fami ... ety/browse
Digital-download games (such as Game Pass games) can be shared between various accounts on a "Home" Xbox. Setting a Home console also allows games to be played without an active internet connection.2) If I choose 3 months of Game Pass for PC at 1.00, I then pay a fee to convert to Ultimate game pass for 3 months. Ultimate will allow me to play game multiplayer game online or download titles to play offline
https://beta.support.xbox.com/help/hard ... -home-xbox
If by "offline", you mean no online multi-player (or other online functions), then I suspect that might be restricted with parental controls. (Unless you disable the internet connection every time your son wants to use the Xbox.) Game Pass Ultimate includes Xbox Live Gold, which is what allows online multi-player capability.
Many games will have an ESRB rating, which helps describe the type of content in a game, and its recommended maturity level.3) Is there a good choice of games for kids.. ie sonic, lego etc
On the Game Pass games list, you can first filter by "Console games" up top, to only show Xbox One games. Then along the side should be another filter for "Maturity Rating", where you'll probably want games rated "Everyone", and maybe "Everyone 10+".
I'm not real sure about specific recommendations. I feel like Minecraft is the obvious one to point out. I don't know much about it, or how much it depends on online functions.
Untitled Goose Game might be good for a video game beginner to get a handle on basic controls. Though it might get a bit challenging, even getting past the first area.
Banjo-Kazooie and Banjo-Tooie are games in the vein of Super Mario 64. (They're re-releases of Nintendo 64 games from 1998-2000.) Those games were probably a beginning point for a lot of people, as far as exploration games in 3D worlds. But given they're 20+ years old, they might feel a bit dated. Banjo-Kazooie: Nuts & Bolts is more like 11 years old, and includes building vehicles out of different parts, which could be fun in a Lego way.
Or there are a few straight-up Lego games. LEGO Star Wars III is based on The Clone Wars TV series. There's also The LEGO NINJAGO Movie Video Game. LEGO Worlds seemingly has some open-world building to it, though like Minecraft, I don't know how much it depends on online.
Super Lucky's Tale is probably good for a kid, but maybe not a great game in general.