he races upstairs to stare from the window and barks frantically and constantly while I'm out thereMy first thoughts to start with in stopping this behaviour are to block off his view, can you fit a roller blind to pull down so that he can not see you when due to go out, if he can't see, he can't get excited and bark for longer than a few minutes of irritation to begin with.
Is he at his adolescent stage by any chance? Often pups are fine with chicks when younger and for the first few months and then their behaviour can change, to then settle down again when older. He does sound as though he needs more training but if an adolescent it is understandable, everything will be a game right now and his testosterone and natural instincts will over-ride everything else.
dog trainer that apparently specialises in acclimatising dogs to farmyard life--no disrespect to anyone, but wasn't sure if this was a 'let the tup threaten to kick the bejesus out of him' school.
At least you are aware of these type of trainers, he may be worth a shot, only use him by recommendation and make sure that you give him a good grilling about his techniques.
In the meantime, sounds like a good training class would not go amiss to encourage those basic commands, sit, stay, down, leave, to heal, recall, quiet! etc, if you are not able to get to classes then a good book will teach you the basics, you need to get going on that, once he is obediant you can then slowly re-introduce him back to the hens. With commands and the hens becoming part of everyday life he may just learn to ignore them or at least obey the leave command.
how I can help him realise that hens have to be treated with respect? Dogs don't understand respect, they understand a command, if he does not comply then he is taken away back to the house. There is no quick fix here, it is going to take a lot of hard work from you to train him to do as he is told, if a dog is not told what to do, it will act on it's own instincts, sometimes that is perfectly fine, but in your case it isn't, well, not unless you want no hens.
swimming/water retrieves and training to work. Just noticed you are training him, then keep going with it, in the meantime, rollerblinds.