KC - if it works when you can't supervise then it has to be a good thing. Would he be distressed by a muzzle?
Things are a bit different with Luca because of his start in life so guessing that you have to do things very slowly and carefully to keep his trust and confidence. Training more commands will probably help in lots of ways, him and you, when you feel the time is right.
DP - My father trained gundogs and I think that once your lab has had a little look at your new hens then that is it, he should be taught that they are none of his business. Dads dogs pick up/trial etc but poultry are nothing to do with them and they have to leave. My sister has 2 cocker spaniels and used a gentle approach letting them 'look' endlessly at her pet rabbits. So that is what they did forever more! They were obsessed!
If your lab is obedient, you could sit him near the hen coop. Give stay command. Walk back, just 2 steps to start with and call to you. Then increase distance. Sit him behind the coop, just to the side, stay command, then recall. He has to come and ignore the hens that are between you and him. Ideas are just to really instil in him the message that you are in command .... hens or not. It's 'upping' his obedience so that he ignores the hens and knows that you are in charge whatever the distraction. Of course plenty of praise/reward when he does well.
If he doesn't know all his basics then it's those without the chickens as distraction to start with.

Cross posted with SITN. Think KC could be right in that maybe the hen fluttered in his direction. We had a blind Flat Coat who could still manage to locate and pick up healthy pheasants from a hedgerow if he 'dropped' on one!

My guess is that at some point he will manage it again! Think SITN is right in that some dogs don't like things over their faces but you can try it and read from his body language whether or not it is a possibility.