Distract him with something like a favorite toy. Lead him away from the chicken. If he tries to go back to the chicken tell him 'leave it" (assuming he knows that).
Tomorrow I would start by getting something that smells like the chickens. Whether it's a feather or an object from the coop. Keep him on a leash. Let him smell it but not touch it. If he tries to touch it say 'leave it' and use the leash to keep him from touching. Reward him when he backs up and leaves it. You can either use a treat, a toy, or just praise. What ever works best for him.
Repeat this on a daily basis