Floppy bunny sydrome is purely caused by a potassium deficiency, nothing else. It is more commonly foound in babies, and if you feed tomatoes each week, as well as good quality pellets, I highly doubt that is what he has. Also, surely Holly would have shown signs if it was due to a lack of potassium. It also from what I gathered, affects the legs.
As for the cold, buns are pretty resistant to it, unless the hutch is in a drafty place where rain can soak them, or they have insufficient food in cold weather. It will not cause FPS though.
I would also guess lack of space could easily cause temporary paralysis, as in hamsters. What size is his cage?
From what you described, it sounds more like wry neck (head tilt). Is his head still twisted to one side beth? Your vet doesn't sound like they have a clue, if it is a head tilt, feeding tomato juice will be of little use. Also, feeding apple baby food, is of no use if he has FPS anyway, is that what the vet recommended?