I dont know if anyone has gone through something similar and can offer advice. But my son is 6 years old and he wont eat dinners that arent: pizza, chicken nuggets, tacos (just meat and cheese), or spaghetti.
When I try feeding him anything else, he will just sit there until he starts crying and begging to go to bed or he will make himself throw up the food if we try feeding it to him.
I just want him to eat. No amount of trying to explain that chicken is in chicken nuggets or mashed potatoes are just fries in a different form have helped.