How do I help my 15 year-old son start a business?
You don't say whether your son is interested in the web, programming, or marketing over the web. If he is, then find a part of this he can do, and give him that. Find a smaller piece that can give him a much higher likelihood of success. He'll know when he succeeds, and he'll know too if he hasn't succeeded.
You don't say how advanced you are on this, but if you're at the beginning, then maybe you and your son can investigate turnkey website store sites like those at Yahoo, or other vendors, and you can give your son the task of getting that store site up and running--if he has that interest.
And finally, congratulations. You have a great attitude and you're on the right track. I speak from experience in this area: My son programmed the internet website that took us from $5,000 a month direct sales to more than $250,000 a month, but starting when he was an undergraduate in college. My daughter is CEO now, of a company that earns $10 million a year. But go slow. Give him a real chance to make real success at each step, and keep the steps small.