Here's the fundamental difference:
Apple stuff is made only by Apple, and they have the philosophy that any hardware they sell should be able to do anything that the software on that machine can promise (including video editing with iMovie, for example).
Because of this, their stuff is more capable than the average Windows laptop, but the higher spec also comes with a higher price.
Windows machines are made by all kinds of different manufacturers, at difference price levels, with different specs. You can get a laptop that can't do much more than browse the web and check your email, or you can get one that's equally as powerful as Apple's top range laptops (usually for about the same price).
If your son isn't going to be doing much more than checking email and browsing the web, spreadsheets or typing letters, then any laptop will be fine. It won't matter whether it's a �300 one or a �900 Apple MacBook.
Another difference is the software. Macs come with Mac OS X, not Windows. There are advantages to both systems. Mac OS X is more secure by default, doesn't suffer from viruses like Windows (yet, anyway), and is more polished (what Apple is famous for --- the interface is nice, like how iPods work better than other mp3 players just because they seem to work well).