Hello Kara and welcome to the Disney Parks Moms Panel.
As the parent myself of a child who is sometimes a very fussy, or certainly a plain eater I can totally relate to your very important question about finding menu items suitable for your own Mouseketeer. I'd like to reassure you that in my own families experience of dining in Disney restaurants we've always found Cast Members to be so very accommodating to our child's discerning palate! My best advice is to speak directly with your server and to politely request if your dear son could perhaps order PB&J or something similar; where ever possible Cast Members will do their very best to oblige even if the item isn't listed on the menu.
You may wish to consider popping along to
The Fountain at Walt Disney World Dolphin Hotel because they serve not only PB&J on their kids
menu, but also a very delicious and yummy sounding PB&J Shake too!
Plus I thought you may like to see this recent
Disney Parks Blog - Disney Parks Sweet Treats which showcases a truly scrumptious looking PB&J inspired s'mores dessert...
Wishing you and your family a truly magical and PB&J filled upcoming trip to Walt Disney World Resort.
Have a great Disney day.
~ Jane