If he’s a kid some resorts may have a helmet rule. Though it’s more the case in North America.
Out of interest, ask the question in North America and everyone would think you’re daft for asking. It’s just the norm now for most to wear them except for old folk or in posh resorts.
Anyway, my unproven example is a friend wearing a helmet smacked his head on a slab of ice and ended up with brain fluid leaking from his nose. The old “had he not been wearing a helmet” is that back of his skull could have been smashed in and far worse result. Purely opinion and unproven of course.
Me, I’ve had a few occasions of slamming my head down hard on hard slopes, and yes while skiing, and combined with mostly used to skiing in North America, it’s just normal for me to wear one. Plus they’re fairly stylish these days and keep my head warm.
Plus gives you that Enduro look 😉