How weird! My guess on the stinging thing would be sharp fins? Larger fish can very readily cut you with their fins, so maybe his are just particularly rigid?
My moneys on that he passes out and gets stiff, hence the sting from the fins. Cory fins do that--and fronties for that matter--uh, the sting part not the pass out part. Poor little fella. Mabe it's like a seizure?
My guess is that it is a defensive thing! Some other animals play the I'm dead game to stop preditors eating them! Not sure bout the stinging though it could just be like others say the rigid fins pricking you!
Quite interesting though!
Love the fish too they are really cute!
Let us know your findings on the playing dead and stinging!