Kathy,
The problem with short term insurance is that there's an age restriction against it. Only a few insurers will take on board drivers who are 21 of age while the others are ideally looking for people who are either over 23 or over 25. It's such a shame because your son has been driving for 4 years without any claims and it should be easy for him to find car insurance as he has the driving experience as well as no claims bonus to prove it. Have a look at the list of companies for short term car insurance and see if there's any insurer which you've missed and try to contact them for a quote.
Alternatively, you can get car insurance for 1 month for your son under a Pay As You Go scheme which is basically a rolling month car insurance policy. There's no age restriction on this one and your son wouldn't have any problem to purchase it. It's just a few days longer than you need but if you find a reasonable quote, then why not?
