Buy him a used premium business laptop from ebay. Pay no more than £200 / $200. Aim for a deal at around £150 for a fully working laptop.
Current sweet spot for laptops are HP 840 G6, 830 G6 or Dell Latitude 7400 or 7300 or 7410 or 7310.
If he doesn't need a laptop now, wait till July / August 2024 and buy a laptop then. The sweet spot will probably have moved on to HP 840 G7's, Dell 7420's etc.
Linux is the best operating system for your son's laptop. Debian with KDE Plasma is more stable, more secure, less virus prone, more reliable, more polished looking, has a better user interface, is less annoying (eg with updates), cheaper (it's free!) than Windows 10 and 11.
Libre Office is a good free alternative to MS Office.