Sorry, I don't think any 5 year old should know very much about the real workings of a real gun. Only to recognize one, and leave it alone, and tell you or your other half.
Too many sad stories.....bad endings.... plenty of time later when he gets older. When he is old enough to disassemble and clean one, he will probably have the judgement to control those kid type ideas that got us all in trouble.
Now....I still remember how much I loved my Hopalong Cassidy cap guns, my Roy Rogers shirt with the fringe and laces instead of buttons, and my Nichols Dyna-Mite derringer cap pistol.
The Nichols had a brass case and a lead slug drilled through. You had a round single cap that you put between the case and slug. When the hammer fell, the cap would get smashed between case and slug....and then a wisp of real cap smoke would come out the barrel. Cool! Loved it. Kinda simulated loading a real gun.
There were several styles. You can still see them on Ebay. Maybe that is what you are looking for...something different.
http://www.nicholscapguns.com