Ring his doorbell and confront him. Ask him what he claims is so "unhealthy" about living below you. Find out what his problem really is.
Of course, you should definitely bring up the point about your car's transmission dying. Ask him why he assumed that your car is parked in such a manner as to purposely annoy him.
The guy believes that you are intentionally out to get him. Otherwise why would he have talked to his doctors?
Ring the doorbell and confront him. When he's forced to address you face to face it is extremely likely that he will either back down on certain stances or you will be able to better understand why the guy is so damn crazy.