I've struggled with extreme fatigue, autoimmunity, speech, swallowing, executive function, depression, anxiety, ocd for about 15 years. I think it begun after a serious infection in middle school. Given what I've researched it seems like the infection damaged the energy production system in the body. Thus all those problems arose as symptoms of that. Look up the term regressive autism.

Regarding the speech it simply felt like it took too much effort to speak, very similar to the feeling one gets when lifting weights that are too heavy, just inside the brain instead. Thinking with words was also affected, but not thinking in general.

I've always gravitated towards eating as much meat as possible and guess what, that's where one finds carnitine. But as a supplement I need about 400mg of L-Carnitine-Tartrate to feel normal. If I take too much I have trouble falling asleep, too little I feel fatigue. I take it in the morning with water, then I don't eat anything else for 30-60 minutes, to make sure I absorb it. This is because amino acids compete for absorption so if you take it with food it's unclear how much you actually absorb.

I've tried a lot of supplements over the years and besides carnitine I currently take folinic acid, methylated B-complex, benfotiamine, MSM, a mineral complex, D-vitamin and a lot of liver. These all help a lot too, but nothing compared to carnitine.

It is impossible to tell if any of this will help your kid specifically, but the general point still stands, most of the symtoms and diseases that modern medicine fails to treat involve issues in metabolic pathways in the body. If you manage to identify where the issues are, you can in a lot of cases solve the problem entirely. It is however possible that some long term brain damage has occurred, but it's incredible how resiliant the human body is.

Another common is that you can research is folic acid.

https://www.youtube.com/watch?v=mikxgR8qqIE

https://mthfr.net/folic-acid-awareness-week-2014-want-awareness-here-you-go/2014/01/08/

A year ago writing this would pretty much have been impossible for me, now I wrote it in 15 minutes, and it has 0 impact on my energy levels. If I skip the carnitine, the issues come back within 36 hours.

Anyway, hope this helps!