Frankly if your son has been on this diet for awhile and you have seen no results, I would start questioning the doctor a lot!
You also should start pushing for a diagnosis from some one other then a pediatrician. I would suggest a pediatric psychiatrist as first choice. Or maybe that is not necessary based on what else the diagnosis team found. Point being, if there has been no formal diagnosis for ADHD, what the heck is he on that diet for. If he has been diagnosed with with ADHD, then you need a lot more information (such as the book - "The ADD/ ADHD Answer book," by Susan Ashley. )
I also think you should look into sensory integration (or processing) disorder or SIDS. A good place to start is here -
http://www.medhelp.org/posts/Child-Behavior/could-there-be-a-BIGGER-problem/show/1590406#post_7226976
Read the post by specialmom at the end.
I hope some of this helps. You really do need lots more info from this doctor to explain this diet. It could be hurting more than helping.