Mama4 --
If you've searched threads on the glutamate topic here, then you've probably seen plenty of my posts in that regard. We've never done Nutrahacker or any other gene testing to date with DS; we "lucked" and trialed-and-errored our way into effective treatment without it. My introduction to the glutamate connection with PANDAs and anxiety behaviors in particular came via research conducted by Dr. Rothstein with respect to glutamate modulation in anxiety disorders and the glutamate modulating properties of beta lactam antibiotics.
I did some reading on a non-glutamate diet but honestly, given all the naturally occurring glutamates even in fruits and vegetables, it really did just appear to us, for our family and lifestyle at least, to be overly-restrictive. We do, however, eat preservative-free and largely gluten free (rice and quinoa pastas rather than wheat, next to no bread, etc.), and that has been a positive for all of us in terms of weight and general inflammatory issues.
NAC has been a part of my entire family's supplement regimen for years now; DS takes the most given as he has the highest tendency toward anxiety, but DH and I also take it as it has also been studied for effectiveness in combatting flu and other respiratory illnesses. It has been good for all of us, but yes, there are some who don't respond well to it. Perhaps one or more of them will speak out and give you some information that will help you better determine NAC's suitability for your DS.
I, personally, am convinced that glutamate immodulation is a significant player in my DS's situation, though I realize there are a number of more primary culprits for others. All the best to you as you work out the best path forward for your family. I'm sure you'll find it.