I agree w/ the previous poster that the Brown Derby is a marvelous restaurant. I love the ambience and food and go there every time I visit WDW, but I don't know if a young child would love it as much. Not much going on there to entertain the children, but if your son is shy, then it might be perfect for him. My kids are too young to eat off the menu yet, but I hear all the Disney restaurants have good selections for kids, so you'll probably be just fine there.
I didn't care for the 50's that much, but I took my PIL (in their 70's) and they consider that their favorite restaurant. They loved the meatloaf and other foods and my grumpy FIL really enjoyed the server and she made his trip. Our server knew who we wanted to torment and I think they are all sensitive as to who can take it and who can't. If my FIL was in his usual bad mood, she probably would have backed off. But he was having a good day and she really got him good. If you want to keep your PS and try it out (it could be quite enjoyable for him if he is in the right mood), then I would talk to the server and explain your dilemma and I am sure he/she will act accordingly. The server can also focus his/her attention, not on your DS, but on your husband or grandpa or someone else joining you, and I'm sure your DS will get a kick out of seeing his Dad being scolded for wearing a hat, etc. He won't be in the limelight, but can enjoy the antics. That's if your DH isn't shy as well. Ha ha. Hope this helps.