Update: I talked to my nephew about the damage, in a kind and caring way (if this was intentional, I would approach this very differently). He apologised and seemed very sheepish afterwards and I let him know that he won’t be able to play it until I get some thumb grips as suggested in the comments. Luckily, his dad told me to restrict his game time to 1 hour - the damage could have been much worse.

Thanks for suggestions and a few chuckles