Leri, I am not an expert in spaniels but do have quite a bit of experience of dogs including working sheep dogs.
My collies are almost never loose unsupervised and even when they are, briefly, I am always nearby and it would not be for very many minutes.
If it is not practical that your boy could live an active life with you and never be loose unattended (except where he simply cannot reach any hens, either by penning him or penning them, or both), then, if it were me, I think I would try the Dogs' Trust, with a hefty donation to cover their costs in retraining and finding him a suitable home where he can be accompanied when loose at all times. They never put down a healthy dog, they have loads of very experienced dog trainers and psychologists and they are dedicated and careful at fitting the dog to the home. Any that are not suitable for rehoming have a home for life with the Dogs' Trust.
I had to take a friends' dog (a young Springador who had become a handful with sheep) to the Darlington branch and I was very impressed with their professionalism. The intake psychologist took him away for an initial assessment while I filled in the paperwork, including a very comprehensive history and my opinions on what sort of home would and would not be suitable. She returned after about 25 minutes, told me a lot about the dog that was right and a few things that I hadn't thought of myself but seemed very credible, and finished by saying that he was a complex character and she would keep him under observation for a week at least before she released him for rehoming so that she was sure she knew him properly and could advise correctly on the type of home that would be suitable for him.
I left feeling very confident that the dog would end up in a good home and that it had been the right choice to let the professionals rehome this dog who had the one problem but was otherwise an absolute beauty and would make a tremendous companion for the right someone.
I hope a good solution comes up for you and for Max - and also for your family relations and relationships.