Doesnt sound as tho you plan on using him in a breeding program. Am I right?
You need to get him gelded. I would have him in a more confined area and start working with him. He needs lots of hands on and taught some mannners. We had a breeding stallion for 5 years and had him gelded at the age of five. He was very respectful and new his place with me and my hubby. Our stallion was hand bred.
It is more work with a stallion than other geldings and mares. People that came into our barn didnt know the difference from him than the other horses as far as his temperment and that is what you strive for.
Nipping is fighting words at our place. Shouldnt be allowed ever. Sharply crack that muzzle with a sharp "NO" and every time till he gets the message. They communicate with their mares that way but he is to know that "You are not his mare" You are the "LEADMARE" You set the leadership role.
If you are not knowlegdeable enough to handle this stallion then get help, really.
Good luck with him and never turn your back on him.