My son is now 13 and we went through hell for about 9 years - he would not eat any fruit, vegetables, butter, milk, yoghurts, cheese, fish ... the list goes on! He lived on rubbish - chicken nuggets, potato waffles, chips, ketchup, sausages and of course chocolate and crisps! He would not eat these items if there was anything else on his plate! I tried everything but did not want to starve him, even for a day - motherly instinct and he was very stubborn as well.
It was only after he went on a school holiday for 5 days and came home 4 pounds lighter and stating that he now liked tomato soup that things started to change. He was literally starving and tried soup at the hotel they were staying at - that was the turning point! We have never forced him to try anything but gradually he began to request a taste, and now it amazes me what he eats. Only last night we conquered the hurdle of eating chicken that had a bone in it!My son now loves indian food, chinese, peas and carrots, baked potatoes, pasta, fresh basil, sundried tomatoes, garlic bread, etc One thing that I did was to just accept the situation which took the pressure off my son and off the family and not to make a big deal out of the situation or else it just gets worse...No child I ever knew has deliberately starved themselves and if you are worried about a balanced diet then use additional vitamins and minerals. All children are different and there is no right and wrong way. If you can force them to go cold turkey so to speak then great, but it may not work and you could make things a lot worse. Also, my son is NEVER ill and never was ill when he was eating a restricted diet. He is 5ft 6" and looks amazing and he is only 13!
Just dont make a big deal about the situation, go with the flow and as long as the child is not ill or wasting away you will be fine. Not many of these children grow into adults with the same habits - their palates can be very immature and so I am sure it will right