If his reverse sneezing is caused by anxiety, at least there is hope that, as he gets used to the fact that he's now in a good, loving home, and not about to have his world turned upside down, it will stop happening. Good luck in figuring in all this out. As you mentioned it can also be caused by allergies, keep a diary of what exactly he eats and when - including treats and snacks - and see if there's any related pattern of attacks.