Since he has baffled the neurologists, I would do everything in my power to see the best available neuroendocrineologist and an immunologist and a gastroenterologist too. But you need the latter two to be atypical because most would shrug and say "Not my field."

PD has aspects of all the above so it is sensible to look there. The endocrine system is maddening and can account for most symptoms of PD. And problems with nutrient absorption in the GI tract are just as bad. Heck, throw in a rheumatologist, too. Inflammation in the nervous system should be looked at.

If it were me, I would be wanting every lab test available with a focus on minerals and vitamins (especially the B's and the electrolytes) in the system. It is possible to eat a great diet and take pills by the handful and still be short on something vital because you aren't able to take it up or you are missing an enzyme or something. Lower limb problems are particularly suspect when nothing else accounts for it.

Good luck.