That's what I was going to suggest. We've done that -I think we had to get a letter from the school.Educational versions are normally much cheaper. Lots of places sell them, just Google student version. You should be able to get it for £60 ish and just need whatever proof they ask for to prove he's at school.
There's also Office 365 online, which is free.
https://products.office.com/en-GB/student/office-in-education
Alternatively, there's now a subscription model - Office 365. I know nothing about it.