Chris,
If you want to save on car insurance, you will need to get a car with a small engine. Have a look at the article on
cheapest car to buy & insure to get an idea of what I'm talking about.
As for your second question, if you get car insurance with Directline and you become the policyholder and your son the second driver (named driver), both you and your son will earn no claims bonus. Other insurance companies will give the no claims discount just to the policyholder. Having said that, you should know that the no claims bonus earned by the named driver (that is, you son), is not transferrable, which means you cannot use it with any other company except Directline itself.
However, something is better than nothing I guess and if your son intends to have car insurance in his own name in the future, at least he will get a discount by building up his no claims bonus by being on your policy for the time being.
You can read more about the benefits on
Direct Line Car Insurance website.