I have been diagnosed with FM aswell, for almost 2 years now i've been having digestive issues and after a few tests i had been diagnosed with a few conditions, FM being one of them. I have somewhat good news for you, in that switching my diet around to be more like what the primal diet is, has helped me improve my health and take my mind off the negatives associated with FM and other digestive issues i have.
I'm in Australia, where the leading research is being done in regards to FM, a university is dedicated to testing foods and releasing updated results on foods. I can suggest you google search 'fructose malabsorption yahoo group' and join up if you feel like reading about people who suffer from FM. It's mainly Aussie based i think but lot of people discuss issues on there and you can gain some insight and updated info.
I actually feel fibre was an enemy of mine, like you said fibre can aggrevate IBS symptoms, and actually my diet prior to this would have been very low fat and i believe i was fibre dependent. I read a webstie gutsense.org and learnt about how fibre can be your worst enemy in fighting constipation. I recommend you look into it too. It's unfortunate your son has the condition at such a young age, but there is hope out there that FM can be greatly improved, with following good habits in regards to the foods you eat and help steady the digestive system and improve it with things such as fermented veggies (good bacteria), avoiding problematic foods, eating a primal way whereby minimising grains that are known to cause distress and eating safe alternatives. Find your sons personal tolerances.
Btw green beans are awesome and i eat em by the bucketload. All the best to you and your son, its a very annoying/frustrating thing but ultimately cna be a minor thing depending on how you manage it.
Also you will find that evey FM sufferer has their own limits and can tolerate different foods and amounts.