I'm not sure if his MS would be associated with being in Vietnam but if he was sprayed by Agent Orange then I would be suspect of any medical illnesses. My dad was also sprayed with Agent Orange and has numerous medical problems and so do my brother and I; both conceived after Vietnam.
I would go to www.publichealth.va.gov/exposures/agentorange/
Then under "Related Diseases" click on "Veteran's Disease's" or under the heading "Benefits" click on "Disability Compensation" and then read and click on other links within those categories.
It's government/veteran website and will have diseases associated with agent orange, studies being conducted past and present, how to apply for compensation, etc.
when they diagnose MS they don't really have much clinical evidence from tests, only a collection of symptoms.
"Almost any neurological symptom can appear with the disease, and often progresses to physical and cognitive disability" that means that all problems affecting neural transmission could be diagnosed with MS.
I heard of other people who had MS diagnosed, or MS-like symptoms that appeared after an infection. It might be an abnormal immune reaction. there are some known auto-immune syndromes like Guillain-Barré for example that are known to be a lot more frequent during pandemics.