Look for a Ford Fiesta with the reliable 1.25-litre petrol engine. It's a great little car and there are loads about, so your son can be picky about buying one he likes the look of. I'd buy from a private seller rather than a dealer - it gives you chance to meet the previous owner and get an idea for how the car's been cared for. Ask to see evidence of regular servicing and look for one with a long MoT - at this price, a car with a short MoT can be deemed worthless within a few months. Also consider a Toyota Yaris.