Demaestro - 3:55 pm on Jun 14, 2010 (gmt 0)
Are you are a project manager working for his company or are you a developer hired by his company? If you are a developer then your job should be to scope the work and estimate it's cost and time frame, not to help him discover the viability of his idea.
I don't know the full nature of your relationship but if you were a building developer you wouldn't ask him the viability of his coffee shop business plan if he wanted to spend a million dollars designing and building it.
Unless this is part of spec creep, which is an entire different problem then just be fair and warn of him of large costs and upkeep requirements but scope the work and let him decide what is best for his business.