If you ds literally only eats those few items then I can't see a way where the rest of the family eat a well balanced and varied diet without cooking for ds separately. So like pp suggested I'd make a load of mash and freeze. Just to make the day to day a bit easier.
As he eats pasta (all be it plain) I would probably include at least one pasta dish a week for everyone so at least you can use the plain pasta for ds and turn the rest into something a bit more exciting for everyone else. Pasta dishes have the added bonus of tending to be relatively cheap. Things like pasta arabiata, tuna pasta bake, pasta with garlic mushrooms, pasta with tomato and bacon sauce etc.
Similar with dishes using mash. Sausage and mash, cottage pie (or cheaper still make a veggie version subbing in lentils, or mix of both meat and lentils, fish cakes (cheapest using tinned fish but frozen fish, particularly things like pollock are fine for things like fish cakes and because you are mixing with potato you use then eating the fish alone.)
Shakshuka is really tasty and inexpensive.
Veggie curries are also really cheap to make, particularly if you already have a well stocked spice cupboard. Chickpea and cauliflower and butternut squash with red pepper are both popular here.
Look for recipes which over flexibility of ingredients, things like fritata are great for using up random bits and pieces left from other recipes.
If you are eating meat look for ways of stretching it further by combining it with other ingredients. For example a beef casserole can be made with shallots, mushrooms, carrots, parsnips, leeks, pearl barley to stretch it further. Similar with fajitas, less meat more peppers, onions, I often add courgette that needs using up.