Hellenbrand makes sense - sort of.
A 21 year old taking their test now is going to find insurance expensive whatever they get. I'd be expecting £1,700-2,000. £1,500 if you're lucky and in a low risk postcode... if in a high risk postcode then the price could be anywhere from £4,000 to forgetaboutit. If you see a quote for £1,000 or less, then you've probably made a mistake entering the details.
You can keep the cost down by going for "cheaper" cars, there's a reason most 21 year olds drive 5-10 year old Corsa/Clio/Fiesta types, and these are usually a good bed as they're small, cheap to run and reasonably cool cars for a first car. However, while small and fairly slow, they also tend to carry a little more risk than some other cars because so many young people drive them... If they turn out to be expensive for him, some cars like a Nissan Almera tend to be cheaper because they're not stereotypical young driver cars - because most teens/20somethings wouldn't be seen dead in one: they do usually give cheaper quotes though. Toyota Avensis, anything like that. Even larger cars that you wouldn't expect to be cheap because of a bigger engine (Skoda Octavia, Ford Mondeo etc) can sometimes be cheaper because they tend not to be cars you associate with young drivers. Young person driving an older person car = low risk, apparently.