As has been said, scoob, it is the Third Party liability that racks up the premium, not the value of the insured car. (In fact you will find that there is not much difference between the premium for Third Party Fire & Theft and comprehensive cover which covers accidental damage to your son’s own car). Insurers base their premiums on the relative risks posed by different groups of drivers and young male drivers are the most accident prone.
Don’t even think about “fronting” for your son. As has been said, insurers are wise to this and will adjust “your” premium accordingly. As well as this it is illegal and he may be guilty of driving uninsured if it is discovered as the insurers will cancel the policy retrospectively. Apart from that, your son would do better to accumulate his own No Claims discount, which he cannot do if you are the policyholder.
Yes, many young drivers do drive uninsured. I’m sure you’re not suggesting that your son does so and I’m sure you’ll advise him that it is not a good idea. Although the fine will almost certainly be less than any premium he may pay the offence attracts a minimum of six penalty points. This has two serious implication: new drivers who accumulate six or more points within two years of passing their test revert to provisional status and have to retake their test; and insurers will load future premiums even more when the offence is declared.