Be careful about sending you rown foods to be reheated by the staff. Some establishments won't reheat foods unless they are shop bought with instruction on the packaging. (They would have no idea how the food was made, reheated previously etc, so they have to safeguard themselves) Check with your dayare providers for advice before sending your own food in.
You could go for plenty of finger foods and ensure that he has a good meal when he comes home in the evening.
Here are some ideas
Bread, Cooked or Raw Carrots, Diced Fruit, Dried Fruit, Cheese, Boiled Egg, Cucumber, Cold Pasta, Tomatoes, Cold Meats.
He shouldn't lose weight, as long as you give him plenty to eat. Some children snack all day, but I like mine to have at least one full meal per day.
HTH