Has your brother had any blocks. They might try that for the RSD. As for the stomach issue. I was on Methadone for over 5 years and can't take it now due to my constipation and stomach problems.
I was diagnosed with Motility Related Syndrome and Metabolic Syndrome. I am on Amitiza for severe constipation and Metformin for the Metabolic Syndrome. They work good until I have to take something for pain and then I am messed up again which I am now. I had to take meds due to a bout of the shingles and they caused my constipation to get worse again.
He might think about going to a gastroenterologist and have test done where they go down into the stomach and look around and also a colonoscopy.
Has he had any blocks? He might ask about them. He might want to see an Anesteologist for blocks.
I'm glad you are trying to help him. Support is important to people with RSD.

Ada