First, separate the pup from the chickens.
A description of your chicken coop and fenced areas would be helpful. Also, what other livestock do you have?
Chickens are the ultimate squeaky toy. Know and understand that. It may be that this dog never is chicken safe. Did you just let him loose with chickens, unsupervised? What training have you given him so far, how long have you had him and how old?
Along with above, collar and leash....walk pup near chickens. Intro and immediately reprimand if ANY aggression or thoughts of a chase. It won't happen overnight but repeated intro and no chase will teach acceptance.
You will find that prevention and maturity are your very best friends. I put my pups where there is a chicken and puppy proof fence between them. That lets the puppy get over the novelty of suicidal squeaky toys, while his/her proximity and presence serves to deter most predators. Even a dog that seems “poultry safe” as a baby will usually regress in adolescence and find chasing, catching is very self rewarding. They don’t kill stock maliciously, they are just baby dogs in giant bodies. Think Lennie Small.
Maturity usually comes between 18 mo-2 years. Some take longer.
Meanwhile, as @Mini Horses said, praise for calm, ignoring them and correct for undue interest. Always supervise with at LEAST a longline on your pup to prevent mistakes
The time is worth it. Never lost a chicken, rabbit, lamb, or other in 12 years.