Look for something with a very small engine that is about 5 years old or more. Make sure it has never been a write off and is absolutely standard i.e. no changed wheels, engine modifications,go faster stripes etc. But you really do need to get a specific car in mind and then get quotes on it because whilst engine size etc are important there are other factors that matter which you have no way of knowing. e.g. cost and availability of spare parts, is that particular model one that seems to "attract" a lot of claims ? only the insurer will have that info.
It will still be expensive because it is the cost of the damage that your son can do to other people and their property that is the main concern. Unfortunately, statistics show that young men have more expensive claims and at higher speed than the general populace.
Good luck