You mean the game card slot?
Get a toothpick in the slot and give the pins a wipe. Also scrape up the Splatoon game card while you're at it - or take advantage of the fact that it's external to make it easier to clean the pins (use IPA)
Almost always the reasons that connector pins fail on modern devices like phones or games consoles is dirty pins. They can look quite clean but stuff takes dirty contacts a lot less well than they used to - well unless you remember **** like the NES.
Of the things that do fail on Switches, the game card slot isn't really one of them. They're pretty durable, mechanically. So unless your lad has knackered it without admitting to it somehow, 99% it's going to be a bit of dirt on either card or console.
Thanks I'll try cleaning the pins. I think he's tried different cards so I'm guessing it's the card reader itself.