The approach to autism that I have used for 12 years now, even before becoming a Defeat Autism Now and MAPS practitioner, emphasizes nutrition. In my opinion, what you see below can be applied to any mental health issue, child or adult. To go further, in my experience it's a plan for chronic disease in general.
If we see autism as a multi-organ system problem, we begin to understand the importance of a holistic, biomedically-based approach. If we then understand that the underpinning of a solid biomedical program begins with influencing dietary factors, the options before us expand dramatically.
With options comes confusion and complication. Which diet? Which foods? What to cut out? Any program needs to be individualized, and food allergy and other testing can help in this regard, but a simple approach begins with the pyramid below. I devised it long ago as a means to communicate to parents the priorities, and to remove the complexity.
As with any pyramid, start at the bottom, go at a pace you feel comfortable with, and work your way up. Observe your child for changes at each step, and proceed as high as necessary, or as you wish to go. If you wish to start at a higher level, then do so, but I strongly recommend that any lower steps not be skipped. I hope you find it helpful.