Maybe that is just a factor of his age and lack experience at what is possible.
But the question is: Do I try to convince him by showing/explaining to him that his age peers are doing x while he is doing y? Do I try to give him the perspective of comparing himself to others? (Normally, given an "average" kid, I would never think of doing that! But is that what is healthy in this case?)
Instead of comparing him to peers (which probably wouldn't work with him anyway), why not try to give him the scope he is clearly lacking in possible/impossible?
Yes, a rocket brought men to the moon. How many people working together did it take, and how long did they work to get there? Although good luck instilling a sense of time ("decades of work!") in a 5 yo
Too many of the history of science books focus on the lone (misunderstood) genius toiling away in his lab/garage/barn. Especially the ones targeted at kids. But others might be able to suggest appropriate books about things like the Space Program, and other large scale efforts?
Right now DS7 and DS9 are listening to the audiobook of _Bomb: The Race to Build--and Steal--the World's Most Dangerous Weapon_ about the Manhattan Project (
http://www.amazon.com/Bomb-Build-Steal-Dangerous-Newbery/dp/1596434872). Lots of scientists building on each others' work over two decades, plus war and spy stuff, but it is a Newberry Honor book for 10yo/5th grade up, so not too gory.
I especially liked its take on Robert Oppenheimer, from bright genius kid with no social skills to forgetful college professor to cat wrangler of a team of the brightest minds in the world at the time
