What Causes Loss Of Taste In Mouth?

My son can't "taste" flavor, and texture has become the only thing he cares about in food, this means that things like vegetables and other such things are unbearable to him due to their awkward textures and that most times he's out with friends he can't really enjoy food all that much apart from occasionally due to a focus of texture. He's demonstrated mildly to me what it's like to not have a sense of flavor by closing my nose before I eat and then releasing it, but he seems to have an oversensitivity (though perhaps not) to texture because of the lack of the other sense.

Most people think it will pass but this has been going on for years and no one's had any ideas apart from vitamins that don't seem to work.