I once saw a British documentary show about a boy who didn't eat anything but fries. His parents didn't know how to change his eating habit (he was about 4-5 years old). Turned out that when he was 2, he got meningitis, and he was very ill for a long time during the time when his eating habits were supposed to be forming. As a result, all food seemed scary and foreign to him. When the parents tried to make him eat normal food, he would freak out.
The expert started by placing bowls of mystery goo in front of the parents and asking them to eat it. They understandably couldn't. He then explained that normal food is like this mystery stuff for their son. He said that they were making a mistake by not letting him play with the food and by forcing him to eat normally. He encouraged the child to touch the food, smell it, rearrange it and do all the things that kids aren't supposed to do. The parents did this for a few weeks and he started eating.