Reverse sneezing is harmless. But, in saying that, if he is having more frequent episodes, there could be an underlying cause such as allergies. You can stop it by rubbing his throat slowly when it happens or by closing his nostrils for a few seconds.
Here is a link about it:
Reverse Sneezing (Pharyngeal Gag Reflex) - VeterinaryPartner.com - a VIN company!