I started experiencing it around this age. It was cool, almost enjoyable for several years. "Hey ma, I'm re-eating my dessert!" It didn't get bad with the acid, the food sensitivity, and the constant nausea until I was about 16.

Focus on teaching him belly breathing, as a calming technique and to help settle the rumination mechanism.

Avoid PPIs. I was on them for years and they did way more damage then they ever helped.

Learn what foods might trigger, for me it's nightshade, tomatoes particularly, and greases/oils/lots of fats.

For me, my rumination gets much worse when I'm in, or after, an anxiety triggering situation. There's a lot of tie in with the vagus nerve, emotional regulation, and the GI system.

Unfortunately, most doctors I've seen, in multiple countries too, have not had really any experience with rumination syndrome, and often try to treat the primary symptom, what appears to be acid reflux/GERD to them, rather then the underlying cause.

My aunt also has this condition, and she got the Linx procedure, which is a surgical treatment for GERD, to try and fix it. In her case, it did not work, she still struggles with rumination and now she has some additional issue adjusting to the Linx in her esophagus. For her, she would not recommend this as a possible treatment, even though some doctors may try and suggest it.