Did you write a general note, or put the specific issue on every ADR? I think you would need to do the latter, with one note, they are not going to inform your restaurants, and I think having the issue listed on the reservation itself calls everyone's attention to it.
I think the bigger issue is that TS places are limited to what special requests they can honor.
They can only really be expected to create dishes from ingredients they have on hand. While most TS places have bread, cheese and butter to make a grilled cheese, they don't always have white bread or yellow american cheese.
Expecting them to have those items specially sent to the restaurant would be very difficult since they already have a system for ordering the foods they prepare and may not have a system in place to order other items like white bread. Also sometimes these places only place orders weekly so they would have to be alerted at least a week before.
The only other way for it to happen is if a kitchen staff member personally takes control of your ADR and sees that in two days you are requesting a grilled cheese and they would then have to make sure to purchase the items from a regular store, bring them in and stash them somewhere to make sure they din't get lost.
OP I am in exactly your situation with my soon to be 21 year old son, though not as bad as your DH. He too started at 3 but at least he has added a few new foods over the last few years like hamburgers and even cheeseburgers. I nearly fainted when he tried wings last week, on the bone even.

He usually only eats breaded chicken.
He has a limited list of foods to eat so I plan our Disney Meals accordingly. We don't do buffets because first he may not find anything to eat and second he sometimes eats very little (though I'm not quite sure how he can be slightly overweight).
Last trip instead of doing a real breakfast buffet, like Boma where he eats two bagels, for our last meal before heading home, I picked WCC because my DW, other DS and I could get the skillet and he just got pancakes.
Also he eats at odd times so I knew when we ate at the CS in Mexico he could get a hot dog up ahead in the Refreshment outpost and he was happy. Love the
UT app with menus.
