Hi, Amy!
When on the
Disney Dining Plan, kids who are ages 3 - 9 must order from the kids menu at Table Service locations. There is no way to pay an "up-charge" to adult meal. Your best bet if you want to avoid your child eating from kids menus for your Table Service meals is to book buffets or family style all-you-care-to-eat dining locations. In these restaurants, your child is free to eat whatever your son chooses.
Some of my favorite dining options that might work well:
-Garden Grill (family style character dining) in Epcot
-
Whispering Canyon Café (family style dining) at Disney's Wilderness Lodge
-
Cape May Café (buffet) at Disney's Beach Club Resort
-Hollywood & Vine (character buffet) at Disney's Hollywood Studios
Things will be a bit easier when using your Quick Service credits as adult/child credits are not tracked so your son can order from either menu at Quick Service locations.
Your other option is to NOT add the Disney Dining Plan when booking your vacation. If it is not something that will suit your family, you may be better off just paying out of pocket for meals so everyone can simply order whatever he/she pleases. I hope you find plenty of wonderful dining for your son during your magical Walt Disney World vacation!
Cathy