I agree, it's good that he eats a good lunch every day, with that and a solid breakfast you can be confident he's unlikely to starve even if dinner gets rejected. I'd give them whatever they'll consistently eat without stress (within reason) for breakfast - so porridge is great, coco pops is fine - hard boiled eggs are good so maybe you could make a batch once a week and he could have a boiled egg and toast/bagel a few mornings a week?
If your DD has lunch with you then again stick to what she likes. Give them both nice big glasses of full fat milk or milkshake made with real milk if they will drink that, they will get lots of nutrients from the milk.
Then dinners is when I would try and gently introduce a bit of variety. The key thing is for you to not get worked up about it as they may pick up on your stress and get anxious around food themselves or act up around food knowing it pushes your buttons. If they reject what you've made, then calmly remove their plates, maybe always have some bread and butter available on the table alongside whatever else you are serving so there is always a part of the meal they'll eat.
Like Atleastitsnotmonday says, I'd build on the things they do like and gradually add a few new elements. So if they like cheese, try cheesy pasta? Then if they find they like pasta, you could try different sauces on the pasta, e.g. creamy cheese with peas and ham, or tomato sauce with blended veg? Dippy eggs and toast could be another good one, and from there you could move towards beans on toast, peanut butter on toast etc? What about sausages or nuggets, most DC like them, you could try chopping up small to start with and eventually they could have sausages, waffles and beans?
None of this is super nutritious and I'm sure would make the organic home-made quinoa crew go
but I believe in starting with baby steps, get them more confident in trying new things and at least expanding their diet a bit. Bear in mind that they may reject one day what they ate the other (they're fickle at that age!) but no big deal, just keep trying and I think you'll find gradually they do increase their range. As a child I survived for a good few years on bread, tinned spaghetti and milk only, and am now a healthy adult who eats anything, so don't despair!