Will he drink a chocolate, yogurt smoothie? You can use organic cocoa powder and you can put the glutamine in it. I'd start with 1 gram and try to work up to 4.
Do you supplement him with zinc? It might improve his picky appetite. [There was a study done on anorexics and zinc sulfate in water solution. If deficient in zinc, the water was tasteless -- to anorexics tested, the solution was tastelss. To the control group (usually the parents), the water had a metallic taste. Once the zinc deficiency was taken care of, the anorexic kids developed appetites. ]