while this is just my opinion on it, i think showing your cards without saying a word means you have yet to act. bad assumption on your part that he was folding. he can't very well call, then expose his cards to you to see your reaction. and it would be retarded to be like "i haven't decided if i'm calling or folding yet... but take a lookie at my nice 5 4 suited!"

but, this is a home game. while i think he was fine, just pick a rule and go with it, its really not a big deal so long as everyone knows what it is.