I've always hated sandwiches, so packed lunches I like which are gluten free are things like sushi, Vietnamese summer rolls, tabbouleh, puy lentil salad or pasta/noodle salad, Spanish omelette. I love googling pictures of bento boxes for inspiration. Cold chicken, ham, eggs, some sausages, crustless quiches, potato salad are also also gluten free. Can you and your son take in flasks of hot food for soups? If you do like sandwiches, Warburtons have a tiger loaf and a sourdough, which are expensive, but apparently the best GF bread in supermarkets.
What snacks are you eating which aren't gluten free? Fruit, carrots and hummus, most crisps, most chocolate, nuts, rice cakes, sweets, cheese, and yogurt naturally don't have any gluten as rule. Gluten free oatcakes are available, as are cookies and both are good, if more expensive. Most supermarkets also have a range of GF cereals. Cake isn't great as general rule, but there are options. If there's something specific then I can let you know if I've found alternatives