@Cheeseandpickle1 You shouldn't be a wreck. Starting school is a matter of joy not despair. Unless it was an enrolment requirement to know how to read and do addition and subtraction before starting school at the age of 4, I would question the choice of school and the comments of the teacher.
You are not failing as a mother, you are possibly failing as a teacher, but are you a teacher? NO , you are his mother.
When your son will be an adult and he will talk to others about you do you think he will say, " oh she was so great, she would teach me phonics and long division every day" no, it will be the cuddles, the laughter, the game, the hide and seek, the jumping in puddles, and standing up for him in front of a bully school system.
You are uniquely you and your son is uniquely him. You can't compare others. He needs some support with school and you will give him that while maintaining a happy house. Slowly you will encourage new food, by adding a couple more on the dining table and asking that he only takes 1 bite, no more every night, and then he can help himself from whatever is before him, but avoid post meals snacks or replacement because he didn't eat.
After tasting and trying the same new food a dozen time, a child will like it.
Fried fish is also nice. Beer battered is my favourite and the alcohol evaporates so it is safe for children. 1 cup self raising flour, 1 bottle of beer (minus some sips for you), mix well in a bowl and have dive the fish into the batter and straight into the pan with hot oil. Make it a joke about the beer and say it is a secret , so you are going to call the dish, the "secret fish" and make the sign of locking your mouth with a pretend key.
Another fish dish which I have never seen a child refuse is the sole (which is impossible to find in Australia). A pan, generous knob of butter, and lay the sole in it, flip the other side, slat and pepper. Delicious and buttery.
New food, new school are all exciting moment for every one. Why not replicate the school at home if you have one of these blackboard with chalk. Put small chairs in from, with stuffed animals on them and you as well, and he is the teacher . Or play restaurant, and if you have a toy food set, use it, if not, use the real thing, a fancy table, multiple cutlery, a waiter with a menu written together to practice letters, and exploring new dishes you might later suggest you do with his help.
Have a good time together, that is what matters, not that is is the last of the class. After all, doesn't it mean he can only progress up grin ?