Make enough chili, bolognese, and chicken to last at least two days. That way you don't have to cook every day, and the ND teen eats what he prefers.
Stop buying fresh fruit apart from apples and bananas. Buy frozen fruit that can be made into smoothies.
Stop buying fruit juice. It is expensive and all it supplies is sugar. Coca-Cola would be cheaper and not much worse.
Buy large containers of yogurt. They can help themselves to yogurt in a bowl. Yogurt can also go into smoothies.
Stop buying ready meals. They're expensive.
Buy more eggs, tinned tuna, tinned sardines, tinned salmon. You can make fish cakes, tuna salad, eggs various ways, sardines with toast or pasta, etc.
Serve more beans and lentils.
Buy cheese, porridge oats, milk, and cut back on bought desserts and sweet snack items.