I would not worry about your dog getting the taste for chicken, as a few years back my dog attacked and killed 2 of my chickens, I was distraught, after a while
I realised I was the problem, as I would panic each time the dog got out while the chickens were about, he gathered it was the chickens I was paniced about so was
protecting me... one day I thought enough was enough so I took him in the garden with me and stayed calm, each time he looked at a chicken I told him calmly to leave, and since that day he has not even attempted to touch a chicken.....I trust him 100% with them now,
And about your cockerel, its hard to say if he will get better, it may be because he is young and a little in-experienced.... my parents had to get rid of thei cockerel as he nearly killed one of their hens doing this, thank god after some TLC she pulled through, we think he just took an instant dislike to her, as it was her he was only rough with... or maybe she just did not do as she was told, as she was a little skitish.
Hope things get better for your hens, and he becomes a gent