All of these therapies would be calming inflammation and oxidative stress (both related to each other and to stray/excess electrical charges in the wrong places or out of control).
The magnesium I would use is "Magnesium Breakthrough" by "BIOptimizers" since it contains a large variety of salts. The bio-availability and especially the preferential uptake into specific organs...of Mg seems to be dependent on form, or salt.
And MIT students, a few years back, invented Magnesium-Threonate is also of therapeutic use as one possible tool, since it increases synapse density and neuroplasticity. Giving the chance of a brain to basically re-wire itself can be helpful when it is not functioning properly.
Lastly, fish oil, not snake oil, is going to help the body make the best membrane-encapsulated neuron bodies so that the brain is shuttling its neurotransmitters (which are basically signaling molecules) in a smooth and balanced functional pattern.
Other areas for YOU to RESEARCH before purchasing anything are these two things: Probiotics (like Seed DS-01), and Light. Look for a red and infrared light for winter and even summer use. It modifies the power of mitochondria. See this study: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3179978/#:~:tex....
It always helps to be the doctor but if you can't be for lack of time or dont have a naturally discerning mind, then get an opinion from a hormone or sports clinic which is designed to optimize, not a family doctor who wants food on kids' plates rather than competition with supplements. And your kid does need to eat food too, as a base. It might just be that doctors have an inborn instinct to protect their valuable secret knowledge from falling into three wrong hands like a Google algorithm unless you have the right keywords, then you'll be rewarded with the right knowledge. I'd like a conversation of such insights as you might have about this and autism, given the fact that also when I was a child, I felt like I was pretty much just watching the life around me and taking notes, or not developing attention and communicating with kids my age in school or making friends.
If you want your kid to talk more but they have autism, maybe introduce from an interest of their's to a conversational style designed to be technical and teach the vocabulary of the thing that they like learning about.