A recent response to a parent asking about dietary interventions:
Dietary interventions didn't work for my son at all, and his ABA team -- with more than 20 years experience working with early intervention/severe/non- or limited-verbal autistic children -- say they've never seen it make a difference, and that when people do say it makes a difference, it is usually because the child is also in a behavioral program, or that while the child's developmental path was delayed, it was more accelerated than other kids with autism. [Yay for run-on sentences!]
However, I do have many friends who swear by dietary and DAN! approaches -- in many cases because their kids had food sensitivities that made them uncomfortable, which resulted in exacerbated behaviors. So, you might as well give it a try if you have the energy to do so.
Before then, I would reach out and ask for personal stories of improvement. Do not believe anyone who gives you the Jenny McCarthy "Mommy Instinct" explanation. Especially don't believe anyone who thinks they can change your mind by yelling louder than you. You want documentation.
If you're going to try diet/supplements, be thorough and methodical and record everything. I found Google Docs spreadsheets to be an especially excellent way to do this, as you can share data online with team and family members.
Most importantly: Don't let anyone talk you into anything you perceive as sketchy or scammy. Far too many people take financial advantage of autism parents' desperation.
Two different perspectives on autism and alternative approaches:
The Fleecing of the Autism Community
Woggle Bug: Blogging About Autism
Please make sure you're fully informed about autism/vaccines:
Vaccines and Autism: A Deadly Manufactroversy
What do Those Vaccine Studies Actually Say
P.S. I have an extra copy of Special Diets for Special Kids if anyone would like it, as I can now make GFCF recipes/food choices in my sleep.