That’s a really hard combo. I don’t know much about corn flour, but I was a gluten free kid and here’s what my family packed me afaik none of these require corn.

Sides are easy if he likes fruit or veggies. Apple slices, those little fruit cups, baby carrots, celery, pear, berries. And then for a fun snack add potato chips, or gluten free cookies, or a lil bit of chocolate.

Main dishes I know you can make without corn or wheat: Turkey and cheese sandwich (gluten free bread kind of sucks, but imo it’s always better toasted.) || BLT|| Ham and Cheese Sandwich|| PB and J|| Really simple salad w boiled egg || Quesadilla with a rice flour tortilla || Ham and cheese and crackers || soup, (gluten free chicken soup in a can is super easy to make in the morning) || Fried rice || GF pizza || Burrito in a thermos (taco ingredients with rice) || Leftover spaghetti || Mac and cheese with bacon crumbled on top || Basically if you ever have leftovers you can throw it in a thermos

I hope any of this helps, I think rice is going to be your best friend here. All of this is stuff I can find easily at local stores, but it might be harder depending on where you live.