Hi and welcome,
I honestly don't know what might happen because it completely depends on the neurologist he ends up seeing when he gets to the US, its not actually uncommon for dx MS US citizens to change their neurologist for what ever reason and they have to go through the diagnostic process all over again with the new neurologist.
We've heard many varying stories over the years, people having a preexisting dx of MS for years, been on different DMD's over those years etc and when they've seen an MS specialising neuro they've been told they've been misdiagnosed and the new neuro won't continue to prescribe their DMD's, others have had their MS dx confirmed but the neuro wants to change their DMD to one they've already tried......
It honestly depends on the neuro and because of the unknown outcomes, it would be in his best interest to request the assistance of his current German neuro in helping him arrange for one of the MS clinics in the state he'll be staying, to take over his medical care and MS treatment plan whilst he's in the US for an indefinite period. It would likely be helpful if you provide your partner with the contact information of your states MS clinics which he could pass onto his own neuro, who could then contact on his behalf and or provide him with a letter of introduction and what ever necessary paper work he'd likely need to take to a US MS clinic.
Sorry i couldn't be more helpful......JJ