Not completely clear to me based on your explanation, but I'd expect that one of three things could have happened (if I'm reading this right).
1. He prematurely, intentionally exposed his hand in the middle of the hand. His hand could have been killed by the dealer and the pot awarded to you.
2. House decision was to let the remaining cards be dealt with no further betting.
3. House decision to let the hand continue normally (as it did).
A lot of this comes down to house/host decision. I've seen all three things happen before, so you're stuck with the house decision. I think you could have made an argument for option 1 if you really wanted to. Even with the action heads up, unless specifically stated in rules, players are not to reveal their hands prior to showdown. However, in all fairness, he could argue back that it was an honest mistake show the strength of his hand and likely get agreement. If that was the case, I'd at least request option 2 then. Stop the betting, play out the cards as if you were all in and go from there. Option 2 is a nice compromise that most mgmt would go with if the floor person is reasonable.