Short answer: Yes.
Long answer: As the implementer for the project, you need to understand as much about the product as possible. This includes market and technical conditions, which you may not understand as well as the client. If you believe it will fail, you may subconsciously not work as hard.
Explain your concerns to him:
- Start with facts first, like "Similar products like X, Y and Z have not done well. Company A folded last year with this product." Make sure they are facts beforehand, too.
- Then tell your story "I am concerned this project will also fail". Delaying your story means the client may deduce your concerns before you get to them, making them less threatening and more persuasive.
- Ask the client to tell you why you are wrong. Create a situation where the client is safe to discuss this. Use a contrasting statement to clarify your intent "I am not trying to make this fail. I am trying to ensure we are successful".
Lastly, consider approaching the problem from a different angle. For example, would using an off-the-shelf system as a prototype be useful to try out the market or idea?