What exactly are his symptoms and when did they start? Have you already identified some culprit foods?

It is hydrogen breath test with fructose for fructose malabsorption and is considered as reliable test. The fact is that sometimes other disorders co-exist with FM, like lactose intolerance or small intestinal bacterial overgrowth (SIBO) that would require additional tests, but in any case he should have only one test at the time. FODMAP diet is quite strict for 9 years old, so, if you assume he has (only) fructose malabsorption, he should have a low-fructose trial (not necessary strictly fructose-free, and he could eat dairy and table sugar) trial, as described on the bottom of this article:
https://www.healthhype.com/low-fructose-diet-in-fructose-malabsorption.html

Idea is to eliminate foods high in fructose and sorbitol. Here is the chart that can help you:
https://www.healthhype.com/nutrition-guide-for-fructose-malabsorption.html

During three days he should not eat:
– honey
– any fruit or fruit product
– any food containing SORBITOL (in many artificially sweetened packaged foods or in diet soda or other drinks) or XYLITOL (both may be in sugar free chewing gum). Sorbitoil on labels is often denoted as “POLYOLS” that comprise sorbitol, xylitol and other “alcohol sugars”.
– any food containing HFCS (in many drinks and commercial foods, check labels).

If your son has FM, he should experience a significant improvement after 3-4 days of low-fructose diet. In this case it is recommended to continue with this diet for 6 weeks so the small intestine can “rest” from fructose a bit. After that not-so strict diet would be ok, like he would be possibly eat bananas, oranges, berries and other low-fructose foods.