Have a look at bento ideas, stay away from the Japanese children's bento, that will only depress you but basically 4-5 different things.
Do 1 protein, 1 carb, 1 fruit, 1 or 2 veg,
So bread sticks would be the carb, will he eat hummus? That could be the protein.
For fruit 2ither cut up or peel or put something like grapes.
Other ideas, meatballs, rice cakes, rice balls, satay or tandoori chicken.
Sorry missed the thing about the food flask.
Hot dogs? Just the sausage send the bread seperately.
Meatballs in passatta and add uncooked pasta, by lunch it will be cooked.
Tinned meatballs in sauce - easy to do if you have run out of other options.
Will he eat stew / casseroles?
Hot water/stock and have some noodles and veg he can add to make a home made type pot noodle.
Cous cous with veg.
If you can heat the thermos then you can add 'dry' foods to keep warm eg chicken nuggets or even a burger.
Will he eat tinned food? Beans, spaghetti, ravioli, sausage and beans. Lots of fussy eaters seem to like things like tinned food, no idea why.