Your dog could be a boss over you, still, if he learned leash manners - he will do what he has learned in order to benefit from it. Heeling - is just further stage of the same process. Sorry to say that, but if you ask such questions - you are far away from participating in competition, unless I don't get something. Could you possibly send your video?
We should differentuate a so called PATROL HEELING
and COMPETITIVE HEELING
Patrol heeling involves a leash, your dog pulls it but only slightly and reacts to any changes to the leash, his head forward, tail up, and he moves rather freely on your left. This type of heeling you ask your dog to perform when you walk along the streets, as it was designed for police patrols. For trained dogs leash is not necessary:
In competitive heeling ( that is your test) - your dog should glue to your left thigh, head up and look into your face, his whole corpus leans up, that's why he appears as if he is "swimming" with his front legs:
The reason your dog relapses into prey mode - because you allow it to happen, there's nothing besides the leash that hold your dog to you. I suggest you to involve a ball on rope in your training, your dog would mind that there is the ball in your right fist at your chest first, and would look at you when there is an empty fist at your chest later. Balls represent prey anyway, can be a distraction for something that triggers prey drive.