We have a bit of the same problem. DS7 eats adult entrees when we go out to eat and has for several years. I know that chicken fingers and mac and cheese are not going to cut it when the rest of us, including his sister, are able to choose anything that we want from the menu.
There needs to be a "Junior" category between adult and child for the dining plan and more choices than what they currently have. I know that some children do not have a varied palate, but others do.
Our solution will be to go to a few buffets (Crystal Palace, Akershus, and Donald's Safari breakfast I think are buffets), order appetizers/desserts that we can share with him and possibly supplement here or there with an extra appetizer that will be all his. We really shouldn't have to do that, but if it will make his meals more exciting, we will.