My son was dx'ed at about age 5. The only thing I wish I could go back and change would be for Early Intervention when he was younger.
You will find a million and one diet and supplement suggestions out there. When making decisions, just keep in mind that NONE of these have been successfully proven on a large scale when using the scientific method. That does not mean that none of them have helped any children, but do keep it all in perspective.
In my non-credentialed opinion and parental experience, behavior modification is the ultimate key. Various therapies (in our case speech - for pragmatics, not articulation - and occupational, with a bit of psych) have helped along the way.
The best print resource imo is Tony Attwood's book, with The Out of Sync Child as a close second place. I'd recommend becoming as familiar as possible with the disorder and how it SPECIFICALLY affects YOUR child before implementing any dietary changes so you can pinpoint the manifestations you want to focus on and accurately measure the improvement or progression of those behaviors, rather than throwing a bunch of spaghetti at the wall and seeing what sticks.