I have a similar issue with my Pekin Drake. He will not stop breeding the girls and beating up the other boys. He picks fights with out male guinea and he cannot win them. I have to step in as the guinea, once riled up, will not stop fighting.
At the advice of a poultry specialist vet I tried Melatonin at 2pm each day. I started with 1.5 mg and worked my way up to 3mg. The key is to find a happy medium where he stops breeding/attacking and acts like a normal duck without being sleepy. It worked for a couple weeks, then my young ducks started to come of age and try to breed. At that point the melatonin was not cutting it anymore. Keep in mind that I had 13 ducks all coming into breeding age at once. (Total of 23 ducks at my house)
I seperated him and have now put him on Releaves. This herbal supplement seems to be working. I have started to introduce him back to the flock under supervision and so far so good.
From what I understand the first couple breeding seasons with Pekins can be rough as the drakes are pretty hyped to breed and go after the other ducks. They are supposed to settle with age. Good luck!