Got my nephew a steamdeck

I wish i had an uncle like that.

https://www.google.com/search?q=what+games+do+15+year+olds+play

Cs:GO, Minecraft.

many things depending on what they are into, try setting up steam family sharing and share your library if you really want to give him a library to work with.

shooters do tend to be popular at that age.

looking at my just turned 16 year old cousin's library he seems to play

Geometry Dash, Conflict of Nations: World Was 3,, Fly Corp, Splitgate, Red Dead Redemption 2, All the Assassin's Creeds (mainly Origins), Cities: Skylines, Subnautica, Muck, Counter Strike 2, Enlisted, War Thunder, Battlefield V, Sniper Elite 4, Just Cause 4, BioShock Infinite, Team Fortress 2, Half-Life, Portal 2, Left 4 Dead, Grand Theft Auto IV

If he is 15 why couldn't he make his own account? the minimum age is 13 then he can pick his own games

I suppose its important to know what aspects of games are fun for em.

For example if its multiplayer and working towards common goal you could try Deep Rock Galactic - you shoot bugs (like L4D horde shooters) and mine stuff and work together! Community is super friendly too. Sadly I have no idea how it would work on deck due multiplayer online stuff.

If its building and machinery, check out Factorio or Satisfactory - I would've loved those games as kid. Teaches bit of math too

If nephew likes to read then maybe some adventure games or point & click games. Like Disco Elyusium (might be too mature in some cases but cant say how mature teens are nowdays) or Monkey Island games (loved those as kid myself).

Maybe Hades for some tactical smashing and roguelite teachin patience stuff?

Educate them about Scams and phishing first and how they can lose their account permanently for cheating.

If he likes building stuff and physics, you could check out the Bridge Constructor series. There's a few bundles for the games and they tend to be really cheap during sales, but there's no bundle that has every single title (ie. the Bridge Constructor Bundle almost has everything, except The Walking Dead one). A few of them even have DLC that adds a few more stages. Here's what they've released up until now:

• Bridge Constructor (Trains DLC available)
• Bridge Constructor Playground
• Bridge Constructor Medieval
• Bridge Constructor Stunts
• Bridge Constructor Portal (Portal Proficiency DLC available)
• Bridge Constructor: The Walking Dead

If he's playing fortnite and tf2, he wants to play cs2, rainbow seige, rust. PVP multiplayer games.

Focusing on multiplayer shooters only isn't the best idea. And the Deck isn't the best platform for that anyways.
I'd pick highlights from several genres as appetizers. Like Hades.

Idk if a steam deck makes much sense as a gift,when he's not already into various games. Maybe Fortnite Bucks would've made more sense.
Since you alrady got it, maybe just put some steam wallet on his account and he can pick stuff himself. Asking what a 15 year old is interested in makes no sense, at that age it could be anything.

The old Metro and STALKER packs. Portal 1 & 2. Plants vs Zombies.

how about this
https://store.steampowered.com/app/413150/Stardew_Valley/
https://store.steampowered.com/app/1868140/

The number of choices can be paralyzing. When I was a kid, I got one or two games a year and played them for months. Way way before the WWW existed. Buy a game based solely on box art and pray it doesn't suck. Even if it did suck, I played it because what else was I going to do? Go outside?

There are thousands of free games on Steam, so he should have no shortage of options for keeping himself entertained. You can always toss extra keys from bundle leftovers at him as well. And, of course, he can some to Steamgifts and win some free games. :)

My own sons are 13 and 15. They play Fortnite, minecraft and CS:GO. As far as single player games go they love Horizon Zero Dawn, Assassin's creed games, Just cause 3 + 4, Spiderman, and Batman Arkham games. They also enjoy the ORI games, Cuphead, Lego games, sackboy and random games like Heave ho, Duck game, castle crashers, boomerang fu and hypercharge unboxed. Hope that helps!

I played on mine :Forza Horizon 5 god of war ; mafia 2 ; hades works perfectly : red dead redemption 2
i can suggest Little Nightmares 1 and 2 Sonic & All-Stars Racing Transformed Collection ; Need for Speed™ Heat
Titanfall® 2