What a shame. Poor George, poor you.
Like yourself, I had dogs for years, nearly always rescues. In my case mostly GSD Collie mixes, just the one odd lurcher-type, with a lot of hound in her. Obedience, agility, etc. Quite often got brought 'problem' dogs to help, and from about 8 years old I was always glad to train anyone's dog as I loved doing it so much, so I had a reasonable breadth of experience.
Back then, before I was a farmer so all the dogs were pets, not workers, I found the only way to manage the collie types around sheep was to teach them that
looking at sheep was naughty. Once they've 'locked on', the ears stop working and you won't get them back till they're ready. So I did training in areas where I knew there were sheep but they weren't visible all the time - gorsey moorland, for instance - with dogs under close control (if not on lead then walking to heel; always on lead in early stages of training.) Being taller, I would see the sheep before the dogs, so I'd be ready. The
instant the dog saw the sheep, reprimand, pull 'em away. (You'll know it really does have to be the very instant they see them; a split second later and they're mentally gone!) When dog is walking quietly at heel
not looking at sheep, lots of praise. Repeat repeat repeat repeat. And again and again and again. Eventually, all my dogs would see sheep and turn back to me - looking at sheep nasty, back to mum good. You would still get the odd 'moment' - when a dog surprises a sheep (and itself) and the sheep runs, the instinct is too strong and most dogs will give chase. But in normal walking through open farmland where the sheep are visible in the distance, I would have nice controlled dogs walking nearby to me.
All of which gave me a good laugh when I took dear old Horace (long-haired GSD x collie rescue dog; had been a very talented rounder-upper of sheep on arrival
) on my first sheep farm WWOOF. The orphan lambs ran about at liberty and of course came to greet the new arrivals. Horace turned and ran away, the lambs pursued, Horace knew it was naughty to look and kept turning away, they kept trying to come up and touch noses - bless that dog!
I don't know if that approach will work with your George; I guess it depends on how much it matters to him to please you. With collie / GSD types, you get that as a given, so you can use praise and displeasure to guide behaviour quite readily.
Another tactic which the police dog trainers use (at least, the ones I've met do), is to give the dog an obsession with chasing something else, usually a ball. Really get the dog totally obsessed with the toy (relatively easy with collie types, they're OCD out the box!), and keep that toy with you at all times. When you know sheep may be about, keep the dog playing with the toy.
And my only other suggestion is to have a friend with a video camera with you, rename George something like 'Kenton' and (only after weaning and before tupping, of course, never at times when the sheep are vulnerable to metabolic upsets when chased) give the world some entertainment. (Just kidding!)