You shouldn't be lifting the elbow of a bong sau anywhere near your face, it should stay shoulder level or below. You're supposed to turn as you make the bong.
If his punch is aimed too high for the bong to make his punch glance/slide off your forearm into empty space, you shouldn't be using bong sau. Bong Sau isn't sanely employed as a block for the attack to the face, it's for attacks to the torso. If he's aiming at your face then you should be something completely different, if you're playing chi sau.
If you're not playing Chi Sau and trying to spar, then ask a boxer what to do.
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