Fussy eater in my house which my dm thinks is divine judgement because I was one too.
My appetite still shuts down if you put a huge plate of food in front of me. And I’m still fussy, but it tends to take the form of eating lovely, nutritious food and repeating the same lunch for a few weeks until I get tired (currently I’m eating a lot of poached salmon with a salad of spinach, avocado and edame beans.) I have no appetite if I’m thirsty, but I never actually feel thirsty so I’ve learned to just drink water regularly and I encourage ds to do the same.
He has got trickier with age. I used to rely heavily on hidden vegetables when he was young, but he can tell exactly what’s in something or detect changes in brands and ingredients. I’ve learned to respect his superior taste sensitivity! Even as a child I never had much luck with hidden calories - he just ate less. And he could detect whey in milkshakes.
(Fully agree about MCD - they ruined burgers, chicken and milkshakes for us)
Our gp was never interested. Said he’d eat if hungry (nope), and was delighted he was in no danger of being obese 🙄. I’d recommend getting a nutritional assessment of his realistic food intake and supplement from there. We’ve also had advice from a speech and language therapist and an occupational therapist, who were both helpful to an extent.
Personally I made the decision that meal times would be safe, calm, relaxed, times for family, companionship etc so I don’t push new foods at dinner. Sometimes even the smell of other people’s food bothers him and he asks to eat in another room.
But we do taste things away from the dinner table. We have a protocol for that: pick it up and touch it to your lips. Then to your tongue. Then put it in your mouth and spit out if necessary, Then swallow as tiny a piece as you like. That has given him confidence that he can try things, and it’s been very helpful in situations with well meaning relatives when he can say “I tried that and I didn’t like it”. In school they did a programme of tasting different fruit and veg for a week, and he was more adventurous than many of the kids and quite proud of himself for that.
Occasionally we have a “tricky dinner” of foods he can eat but that aren’t top preferences. And a special dessert if he manages it. (Often something awful he’s seen in the shop and begged me to buy) The deal is that he can leave one element of the dinner behind, as long as he’s tasted it (these sort of rules help him because he’s autistic. It’s not how I’d want it but he likes the boundaries to be very clear). I’ve managed to get three decent dinners into the repertoire, two of which I can now serve weekly by taking this slowly. And it’s helped hold on to foods that fall out of favour, or something like your burger example that isn’t quite right.