I would also like to add that you should notify him in writing that the chickens have been straying on your property and that he needs to contain them on his property. Keeping them on his property may prevent the dog from seeing them as often and attempting to chase them.
Your dog on your property is a pet
Your dog on his property is a predator
His chickens on his property are pets
His chickens on your property are prey
Your dog on your property is a pet
Your dog on his property is a predator
His chickens on his property are pets
His chickens on your property are prey