Second hand Mac

Scrilla

eBay has some decent MacBooks if you catch it at the right moment. This search link shows a MacBook Core 2 Duo 2Ghz for £240. Yes, there are issues, such as battery needs replacing, but doable with your £300 budget.

If you want to go PC, something like Currys will give you a good deal on clearance models if you're lucky enough. Keep an eye out on http://www.hotukdeals.com/ as many users there find good laptop deals within your budget there.

You will get more for your money in terms of performance if you go Windows at the price range you are looking at.

CountryGoose

I've a year old Dell Mini 1012 that was running like a dog with XP....

Did the above at it's running really fast and will do all your son needs, with a great battery life.

Using it at home, buy a cheap screen, keyboard and mouse and you'll have a PC set-up that'll last for 2-3 years.

During that time you can save for a Mac, if you must. I no longer think they warrant the price premium that they used to - Win 7 has nearly caught up with OS X and there are some much better laptops out there for less than the Mac.