Neurologists will not be helpful, all they'll be able to do is compare your POIS to migraines which isn't helpful. If you have autonomic dysfunction then a vascular neurologist who specialises in autonomic dysfunction could be helpful.
For POIS if I were you I would either see an immunologist, endocrinologist, mcas specialist or internal medicine dr, functional medicine practitioner
I don't really see how asking for an opinion about what doctor he should see counts as medical advice, GPs have no clue and its not like he's asking for which drug or treatment to try which would be an issue.