He would likely have to use a snack credit to add fries to his fruit plate, or some CS's let you substitute "sides" for the dessert, or he can just share everyone else's fries, since they usually give you plenty.
I'm a vegetarian, and I've never come close to going hungry at CS places at WDW. Some places don't have much, but it's usually easy to walk to a different CS nearby and bring it back, or share the sides and desserts off my dh and kids, or stuff myself on snacks like stuffed pretzels and Mickey ice cream bars!
Definitely check out the menus here or on
www.allearsnet.com to get some ideas where he might like the choices. One good option for everyone is Sunshine Seasons at Epcot--they have several different "stations" to choose from, such as Asian, sandwiches, grill, etc. plus a great dessert selection. And if you don't see something on the menu somewhere, don't hesitate to ask--sometimes they have other options.