Yep, the all-in bet is actually quite a good move. If you have an open ended straight draw AND a flush draw on the flop, the odds are about even that you will hit a straight or a flush by the river. So you actually have the odds to go all-in. Also, it would be very difficult for him to call if he didn't have a King and it's very likely he would fold second or bottom pair. Even if he has a draw himself or even a K with a poor kicker it's hard to call. You will hit 50% of the time if you do get called and there is the chance you would take it down there and then. And if you're willing to throw half your chips at the hand, you may as well go all the way.