This is a Microshift 11-46 cassette.
The caveat is your current derailleur may not have the ability to reach the largest cogs with out a
Wolftooth link or similar.
You still have to keep in mind the derailleurs capacity needs to accomodate at least a 35 teeth for the 11-46 to work. This likely means that doubles & triples are out.
As far as which chainring to run in the front, it depends on how/where the bike is ridden. If he can climb hills with ease & is frustrated with flatland speed, then a larger chainring may be in order. Similarly, if the hills are hard & there is still a cog or 2 on the small end of the cassette that are unused, a smaller front ring may be the way to go. I see 28's & 34's mostly around where I ride. This is probably a buy & try type situation.
Narrow/wide chainrings are what you want. They retain the chain much better. Whereas "standard" rings benefit from the presence of a derailleur to reduce the liklihood of a chain-off, narrow/wide rings have alternating squared teeth to fit the chain links tighter.
You don't
need a derailleur with a clutch, but it does generally help the situation. I suggest waiting to see if one proves necessary.
FWIW: 1x are a compromise in many directions for a variety of design reasons, not the least of which is the difficulty of locating a front derailleur on a full-suspension bike. You will have a much easier time getting a better total range of gears with a double or triple drivetrain. Albeit at more complexity.