I have had this with a few of my kids, to be honest. They have Aspergers and eating was really a problem, it still is quite bad with DS4, but improving.
I think that I may be a bit cruel and old fashioned, but what I do is make just one meal for all the children and not offer any alternatives. Anyone who REALLY can't eat it and has life threatening hysterics gets a bowl of weetabix and a piece of fruit, nothing else. I find that over time, they eat more meals and less weetabix, but it does take a while, a lot of determination and a realisation that there are some things that a child may never eat. DS1 & 3, for example, won't eat raw tomatoes, DD won't eat eggs, DS2 doesn't eat salad veg or leafy things.
It used to be that DS4 ate the weetabix about 5 nights a week, now its only about once. Last night, we were having oat steaks with mushroom sauce, pasta and cabbage, once upon a time he wouldn't have eaten any of it, would have had a shrieking tantrum and screamed until he was sick! Last night, he had the pasta, cabbage and sauce. DS2 eats almost everything, now, but when he was small existed for about 3 years on shreddies, sausage rolls and pancakes with peanut butter.
You could try starting by serving something different with the sausages. MY 5 like tomato rice with a little grated cheese.
Cook 2 cups of rice with an onion, 2 tins of tomatoes and 3 cups of water, salt and pepper. Bring it to the boil, cover, reduce heat and simmer very gently until all the liquid is absorbed and rice is cooked. (about 12 minutes for easy cook white rice, about 45minutes for ordinary brown rice.) When you put it on the plate, sprinkle a little grated cheese on top.
We eat this on its own with some bread or tortillas at lunchtime, but the kids like it with things like sausages for tea as well. It is pretty dry, not saucy and doesn't have a strong taste, so is quite a good place to start.
Another easy thing to try is lentil and rice tacos. Cook 1 cup green lentils and 1 cup brown rice in 5 cups stock, with an onion and some taco seasoning, as above until all the liquid is absorbed. Serve it in tacos or wraps with some grated cheese and whatever other things you like, avocadoes, salad, sour cream. Again, no sauce, not bitty, no obvious chunks of veg unless you put them in.
You could also try eggy rice. You need 1 cup of cooked rice, 1/2 an onion, a little garlic and 1 beaten egg per person. Then you can add other cooked veg as you like. I usually add some tinned/frozen and thawed peas.
Fry the onion and garlic until softened. Add the rice and fry about until hot through, Stir in egg, some seasoning and any veg and cook, stirring all the time, until cooked. All my kids love this, even the ones who don't like eggs. The bigger ones eat it with tortilla wraps to make it more substantial. No sauce, no bits, no chunks of veg, pretty non threatening.
Other things that might work are bean burgers, pasta bake (most of the sauce is sucked in), rice and beans, split pea dahl. Try making something new a couple of times a week to begin with and gradually increase the repertoire until you are making varied meals every night. Sadly, in my experience, it is a bit of a long haul, but you get there in the end. In here, nobody wants to eat weetabix for ever