How to deal with him? We should warn you beforehand, the Naysayer is one of the most difficult people to deal with during a meeting! Be patient and prepare multiple arguments that will reassure him.
Or you could try another option: announce a brainstorming session and ask everyone to make a list of the strengths of the project and to then like the ideas of others. What better way to add a bit of optimism to the Naysayers mindset?