I hate to say it, but with his symptoms and this news of how your neighbor lost a bird (okay one day, couldn't walk the next), you should consider that it might be Marek's. Marek's spreads very easily via dander... for it to spread from a neighbor's property would be very common, sadly. That said, since there is no cure for Marek's, if I were in your shoes I would do some investigation to see if there is anything else -- treatable-- that he might have. If you go to this page and scroll down to
"Are there any other diseases that look like Marek's?" and have a read through, see if any of those fit your situation. It might be worthwhile to try some B vitamin therapy, just in case.
https://www.backyardchickens.com/articles/the-great-big-giant-mareks-disease-faq.66077/