This is not a particularly rational fear, but it's one common among naive clients.
As others have suggested, start with a nondisclosure agreement to protect the negotiation, and make sure your lawyer helps you with that. In the contract for the work, include whatever promise of confidentiality you think appropriate, and assign all work (possibly excluding library code or open-source patches) to the client, so they don't have to worry that you'll run with it yourself. You can even consider a non-compete clause, although those are very tricky to get right, so I wouldn't suggest it.
But I think the real answer here is not in documents, it's in handling the client. Point out to him that you get new clients mainly through referrals from satisfied clients. Make it clear that you develop for a living, and don't want to run a service. (One analogy that works for some people is that you sell picks and shovels to prospectors; if you wanted to mine for gold, you'd already be doing it.)
And definitely tell him that his real power isn't that he has one idea; it's that he's the kind of guy who thinks up new ideas and has the guts to go with them. An idea on its own isn't worth much; any new business requires a lot of adaptation and innovation beyond the initial notion. If somebody else steals his idea, it won't matter, because he'll be 3 steps ahead by then. That's all true of successful entrepreneurs, so hopefully it will be true for him, too.