I have a 5.5 year old son diagnosed with severe ADHD and Autistic tendencies and sensory integration. They are often intermingled as in our case. I have not found one true "cure"; however, he is getting better. I tell all of his caregivers that "the more he moves, the better he behaves." Lots of physical activity "brings down his engine." Good activities include jumping on a trampoline (large for outdoors or small for indoors), swinging, spinning in a desk chair, running, jumping, climbing, riding a bicycle, etc. When he is truly out of control I state that his engine is up and he should "bunny hop, crab walk, sprint down and back." These work great when you are in public and he is acting up. Another benefit is to have him chew gum; there are a lot of receptors in the mouth. He received a year of physical therapy for Sensory Processing Disorder through Excel Pediatric http://www.excelpediatric.com. A great book to read is The Out of Sync Child by Carol Kranowitz. Love and Logic is a great discipline book also. I started with Love and Logic for the Preschooler.
In addition to the activity, our son is medicated. He recently switched from Adderall XR to Vyvanse. Vyvanse is brand new on the market and is an 11 hour rather than 8 hour lasting medication. He is also on Risperdal two times per day. Immediately, we saw him focus on art projects, etc. that he had never done before.
I know that you mentioned trying several diets. Have you tried Dr. Feingold's program? It removes artificials and salicylates from the child's diet. We have found that salicylates are not our issue; however, the artificial colors, flavors, sweetners, and preservatives are. The program is well worth the cost associated and you will be amazed when you start reading labels closer. http://www.feingold.org
Lastly, I am just beginning treatment for both of us under the care of a family Psychologist. She will develop my parenting skills for our specific situation. We both met with her the first time and now I will go by myself the next few. I am anxious to receive some support from someone knowledgable on our matters.
Stay strong; you have received some good advice for how to address the lack of medication compliance on your ex husband's part; however, I once heard our therapist tell another parent that lack of structure, discipline, etc. on the part-time parent's part will not ruin the child. You are the primary care giver and your child learns mostly from you.
Let me know if I can ellaborate any further for you!
M.