Since you won't be on the Dining Plan, he has more options. Most picky eater (like myself) can look at a menu and tell what they will or will not like. Therefore, if he doesn't see anything he likes on the menu, with the exception of buffets, just let him order whatever he wants which may be nothing, then allow him to grab something at one of the many Quick Service Dining Places to finish his meal.
You could also have him look at the menus in advance and help you plan your meals, or just take him past the restaurant to look at the menu before the meal, that way he could potentially grab something to consume beforehand if nothing on the menu looks appealing. That way he's not sitting there starving while the rest of you are eating.
When I do Disney not on the Dining Plan sometimes all I eat at a meal is French Fries. Considering Disney food is so expensive, unless you're on the Dining Plan, it's really not worth it to be trying new things.
My younger brother and sister are also very picky. For our trip in July I sent them menus from a bunch of places I was thinking about getting ADRs for. I told them to pick one from each group. (they were arranged by proximity to the park we would be in that day) I threw the Biergarten in there, because I thought my Dad would enjoy it. I didn't think they would pick it, but they did. It was because it had Macaroni & Cheese on the menu.