Hi Nicole!
I certainly wish there was one restaurant where we could see all the characters! I would be happily sitting there just as "star gazed" as your son! :-)
But Disney generally "spreads the wealth" across the 4 parks as characters are concerned. I would assume this is done to allow all guests a number of choices, no matter the park they visit. It also is strategic, as most guests would probably visit that one restaurant and less of the others...
My best recommendation is to go to the Disney World restaurants site here. Then to the left menu under "Dining Experience," put a check mark by "Character Dining." That will filter all the character dining experiences in the 4 parks. Then you can look to book restaurants with the characters that are your son's fave's. If he's a real Mickey and Friends fan, I would go with Chef Mickey's, if it's Pooh and Tigger, then Crystal Palace, and so forth.
Good luck on your planning! I hope your son gets to see and meet as many characters as possible at the parks!