Sorry for the delayed reply!
With my eldest I gave in and fed him what he wanted to eat which was cereal three times a day. At least it was fortified, lol. My Old frowned on this but he was also a terrible sleeper and the food helped his sleep.
Gradually, once the pressure was off, he started to try a few things. He tried and liked rotisserie chicken so I build on that to get him to try thinly sliced sirloin steak and other roasted meats. He doesn't eat these things often or in any quantity and doesn't like any gravy or accompanying veg. However, he now can find something in a restaurant to eat.
Last year, age 7, he tried and liked roast potatoes (he already liked roast beef). When we were on holiday he tried a burger and will now eat that too!! This year, age 8, he tried and liked pizza hut margarita pizza but has tried and doesn't like any store bought. Again, at least he can eat out now and with other people. So... a real success story.
Having said all of that, day to day he eats sausage and beans for dinner with a Weetabix (no sugar) for "desert". Weekends he eats it for lunch too.. so no variety.
But as I said, he is full enough after a meal and happy with lots of energy and I stopped worrying once we left the cereal-at-every-meal phase which was about 3 years ago.
He is just as fussy with sweets/won't try them/won't eat cake or baked goods.
In a nutshell, I just left him to it but applied gentle pressure and ligic to expand a little on the foods he was willing to try. He is athletic and I increased his sports ten fold which increased his appetite and willingness and improved his mood (he used to cry A LOT).
Sorry tgat was long but I hope it helps a tiny bit.
Tell me more about yours. Was the fussiness always there? My ds was impossible to wean so the issue was there from the start but he was already improving by age 6 and I really feel for you that there's no improvement in sight!