Is your son taking only clonidine? Are there any other problems in addition to the ADHD and tic disorder? Does he display any voacl tics in addition to the motor tics? You'll want to rule out Tourette's Disorder. Since you are close to Chicago, it would be worthwhile to have him undergo a thorough evaluation in both neurology and child psychiatry at Comer Children's Hospital at the University of Chicago. You will not likely find much, if any, benefit from alterations of his diet. The two conditions you describe are both neurological conditions and seeking a biological treatment is very prudent.
In our case diet does make a difference. Our son's tics worsen when he is tired or stressed. Therefore we try to make everything as steady as possible for him. Meaning, not a lot of junk food, enough protein, lots of talking through anxiety, and good sleep. We did the Gluten Free Casein Free diet for 2 + years, and through trial and error know that in our son's case too much high fructose corn syrup or food dyes will spur a round of tics. Whether it is because of the food or the disruption on his system I don't care. I've loved my son through six years of tics and will do whatever seems to help!
Recently my son told me that he thinks God allowed him to have Tourette's because he might have been a bully, but the tics have made him humble. Kids have a great way of seeing things sometimes! Best of luck to your family. :)