I have not seen the movie for a long time, and I remembered the character issue with metal rather well, but as far as I remember he just hated the metallic taste.
Little Junior Brown: I ain't going back to jail. I'm tired. They didn't allow me to use silverware. I had to use my fingers. That's right; they didn't even allow me to use plastic spoons, forks, and knives! But I do hate the taste of metal in my mouth. That's why I refused to have braces.
In any case there are some people who are allergic to metal (some claim that 10% of the population have this allergy) and it is mainly affected by nickel. For these people touching metal (or wearing it) is an unpleasant experience, my guess that tasting it is not much better.