I'm looking forward to reading your suggestions....I have the same issue. My son loves hot food and definately isn't a fan of sandwhiches and other cold foods. He'll eat them, but he much prefers hot foods.
One suggestion I can add though.....and I think I read it on here previously. Before putting the warmed food in it (in the morning)...put a cup of water in the microwave and boil it, then pour the water in the thermos with the lid on and let it stand while you prepare the food that's going in there. The boiled water will help get the thermos to the appropriate temp for the food and help to keep it warm all day. I've tried this with my son's thermos (his is a cheap off brand from Walmart) and its working great (as great as can be for such a cheap brand).