I had an issue with a drake, he would bite very hard, I tried many things, including a 'pecking' (finger tips pinched together to mimic a bite) on his beak. Nothing worked.
He was trying to defend his girl, and it ended when I had to put him in a room next to her (separated only by a window) when she was sitting eggs. There was nothing else to do, he will bite if he sees himself as her line of defense.
The funny thing is as much time as I spend with them, it was the men who saw him who immediately understood what was happening, including two workmen who were at the house for just a few moments. There is a male instinct to protect (some men don't have it, of course, and some make it something unhealthy) and a man sees it in other males.
Try to not put yourself on the opposite side of him from his mate (if he has one) to see if it clears this behaviour