I feel your pain. I used to be a chef. I already raised two children who happily ate all sorts of very grown up foods from a young age.
Then I had my 3rd. Who was distressed at the concept of solid food from the beginning. Would have happily stayed on a liquid diet for a couple of years. Then got tonsillitus and developed a deep suspicion of all food. Then we moved countries, so even basic foods tasted different. He turned 2 a week after our move. Spent the next year cajoling and fussing and unhappy at his habit. I never left the house without a little insulated box of foods he would eat. Good advice anyway, but absolutely required as nothing usually on offer for children was of interest. He wouldnt eat flippin' french fries! Or new and unknown biscuits or candy either.
Before a cross country flight to visit my mother, she asked for a list of what he would eat, so she could have them in stock already. Turns out, he actually ate pretty well. Just not what people expected a kid to want. Having the list in front of me made it much easier to accept, somehow. Much easier to work with.
So, i stopped trying. We ate a lot of pizza, and pasta, and stirfried noodles. And plain grilled meats with 2 veg. He ate a LOT of veggie sticks, pizza, fruit. I fed him the healthy foods he would eat in as large a quantity as he wanted. I treated him to the treats he would eat as I felt necessary. We went on like this for quite a while years. I became aware that I was being judged as being a mummy bizarrely strict about the healthiness of my child's food. Bringing veg sticks and cut up plain chicken to birthday parties because I wouldn't 'let' him have hot dogs and french fries. I went with it. Better they judge me, than judge him as being a fussy child because he wouldn't eat what the other children do. Adults harassing him would not help in the long run.
Gradually, we could introduce new crunchy veg. When I was prepping our meal, i just set aside his portion raw. For cooked veg, I became that hide the veg mom. Pureeing extra veg into sauce for spaghetti or pizza, for example. Also into hamburgers! Or figuring out that i could add quite a bit of slivered veg to stir-fried noodles and they would all slither down. Mostly. New raw fruits were easy, but cooked fruits is still a big fat NO.
Gradually, we introduced the one bite rule for new foods. When he was about 6 I think. You dont have to like it, you just have to eat one bite, (often presented on a flat bottomed chinese restaurant spoon, so as not to contaminate the meal) and then say, 'thank you, i have had enough (that food) for today'. And we responded, 'thank you for trying (food), i appreciate your politeness about it.' Took a while to establish this, but it did become a habit.
Three things happening with this. 1. Exposure. You need to experience a new taste a number of times to get past the automatic rejection and actually taste it. 2. Manners. I expect he will ALWAYS have issues with new foods and certain foods, and so, he must learn good manners to deal with it. Nothing worse than a whingey dramatic grown up kicking up a fuss over a food. 3. Control. Sometimes, if he asks politely, he can skip the try. Because i want him to feel he has some control over his food. By letting him refuse, I like to think he sees himself as making choices to try when he doesnt refuse. i dont accept a refusal an actual first time ever try. But after that, I might. If I say 'it's up to you, he usually will eat.
One of his sisters really helped at first by pointing out that it's best to get the try bite over with first thing, while it is still hot. It isnt going to get any better sitting there getting cold. She also taught him to ready a forkful of the best thing on his plate, stuff the try in, chew it and swallow super fast, then stuff in a bite of the good stuff. She made quite a game of practicing this every time she came over. A messy, messy game. Sometimes it was hard to tell she was 22 years older, instead of 2. She taught him to eat french fries though, which was handy as they are pretty universally available.
Quite often, one new food will open a new range. He discovered lentil soup at school, so i have added first lentils, and now other beans to the try bite range. He likes pepperoni, pepperoni is sausage, so he had tries of sausage every week. now he eats sausage, but only Cumberland as that was what we had the night he declared it good. so thats what I tell him they are. Sometimes they are cumberland sausage made with other things though, 
. He likes kale, bizarrely after 1 try! Recently, we introduced flat kale (spring greens) and baby kale (chopped brussels sprouts). Ball kale (savoy cabbage) is on the menu tonight. He doesnt like them as much as kale, but he knows the store sometimes doesnt have any. Recently, a soup made with kale, sausage and cannelini beans was eaten with no discussion at all. 
. One the other hand, he has no idea he has been eating cabbage or various chinese greens in the stirfry noodles all along.
So, yes. Just evaluate his true range, and work with it. Once the nightly battle is gone, set up strategic defense points, and gradually work out. This has gotten us to seven and a half. His willingness to try new foods, and eat things he claims not to like has much improved this last year. Which may be because of my super terrific system 
, or maybe peer pressure at school, or maybe he is just getting too big and active and he is too bloody hungry to restrict himself.