Your specific auto insurance problem is a familiar situation for many families accross the country; but still a very good question. I've found that MOST car insurance companies give you very good deals on having multiple cars being insured under one policy. Of course, you're worried about your son's additional liability being added to your policy. This is a very real factor and it should be taken seriously. If your son starts his own policy, it will NOT be cheap auto insurance because of his age and sex.
Like the previous Answer mentioned, you can have him under your policy with him NOT insured on your car, but only on his own and you'll save money this way. As far as his liability goes, there are many things you can do to keep his record clean. Traffic schools and defensive driving programs will keep his tickets off your policy.
You can also qualify for MANY discounts in your specific situation. (And probably some more than you haven't even thought about). For example,
Auto Insurance - Welcome to Auto Insurance Quotes for Free has a car insurance Discount called the
Multi-Car Discount that you should really learn about. Among others on that site, it will help you with your situation in calculating which auto insurance policy will be most cost-effective for you and your son.