Got a rule question...

So I played a small home game tonight. It came down to me and another guy heads up.

During one hand we had some decent preflop betting. The flop comes. I decide to go all-in to buy the pot. The other guy sits and ponders for a moment. Without saying "I call" or "I fold" he turns over his cards. After he turned them over I grabbed for the chips. He said "no, I called" to which I replied "you can't do that, you never made an official act on my all-in bet, and since you showed your cards without acting while our chips were still in play that means you forfeit/fold the hand." He said "no, I saw Hellmuth(or someone) show a hand before they folded it on the WSOP."

I'm pretty sure I've seen an instance before on the WTP where someone showed his hand while his chips were still in play and he was forced to fold the hand.

So what's the official ruling on this one? Since he flipped over his cards before he officially acted(with no impliction of whether to fold or call) is he forced to fold and I win the hand?

My arguement stems from the fact that if he flips his cards and I see them, he'd be able to see my reaction and have the extra advantage of seeing if I reacted and gave away a tell .

Btw, he ended up beating me...I pushed him all in when I had pocket 7s...he called w/AJ, rivered his jack...then on the final hand, I pushed all in with 2pr, js and 10s(JJ10 on the board, I had a 10)...he called and hit a king to give him a higher 2 pr Needless to say I was a bit perturbed...mainly because I had been playing some great poker(stole some big pots with nothing and sucked out tons of money when I hit my hands).