I don't want to be considered a trainer of LGDs, and most everything I have earned has been taught to me by Erick, and observation of Rika and my other dogs. I did obedience train all my other breeds of dogs intensely. They were on hand signals as well as voice commands. LGDs are completely different. I am lucky if my LGDs come when called if they are after a predator. LOL
First in my mind is him jumping on you. Knee him in the chest, HARD. Like back flip HARD.
Don't wait for him to jump on you before doing the hard knee to the gut since you are not expecting it and cannot brace yourself. Instead coax him to jump on you. Sounds counterproductive, but it isn't. Talk excited baby talk, call his name, pat your chest, call him to you and brace yourself for when he runs up all happy and jumps up on you. Step into his leap and really knee him hard in the gut. He needs to make a strong "OOF" sound. If you can knock him over even better. Tell him "NO" in an angry tone as you do this. Then when he comes up with all 4 feet on the ground bend over and tell him how good he is. Keep doing this until he realizes that he gets knocked down when he jumps. As soon as he runs up and stays on the ground fuss over him big time. You can also enforce the "sit" command when working on the jumping by making hm sit to be petted. I don't use the "down" command for anything except lay down. I use the "off" command but whatever you want to use, it has to be consistent. You can't teach him to lay down with the "down" command and also use it as a warning not to jump on you. It will confuse him.
Also remember that when the sheep run the LGD assumes they are running from danger. If they are running out from the small night pen, the guardian wants to run to get ahead of them in case there is an unknown danger there. It they are running away from him he runs to see what they are afraid of. Since they keep running, he assumes that the danger is still present. Maybe on the other side of the flock or where he can't see it so he cuts through the flock in an effort to get there before the predator attacks. As he gets older, he will learn to run around the flock instead of through it, and he will learn to patrol the field before the sheep are let out in the morning. If he is not patrolling the field at night, this is a good reason to let him out into the field before the sheep come out of the night pen. (Ozel still often rounds up the sheep and herds them back in the night pen in the morning when they try to run out on the field.)
This is a young dog and has been badly trained to this point. In spite of his actual age, you have to treat him like a very young puppy who needs to be trained from the beginning. Work on the jumping on you specifically every day for about 10-15 minutes
without the sheep. Once he has learned that jumping on you as pack master is not tolerated, he will begin to accept other corrections to his behavior.
One thing I noticed in the video is that you called his name a lot. Were you calling him back to you? If not, I wouldn't call his name without a command since he will learn to ignore the use of his name when out with the sheep. You want him to respond to his name quickly and if he hears you using it all the time when you are not giving any command, he will ignore it when you need him to respond.
Why did you want him not to chew the stick? By picking up the stick and chewing on it he was signaling that he was happy to allow the sheep to go about their business and was no longer interested in chasing them. By telling him to drop the stick you were refocusing his attention on the sheep, and he may have read that as your intention for him to chase.
With a shorter lead, it may give you a little more control.
I would take him into the small night pen with the sheep and see how he interacts with them. If he doesn't try to play with them in the small pen, you can try in the large pen again. The long training lead is only used after the dog is responding to commands on the short training lead. Remember also that he needs to recognize his sheep by scent as well as sight. Allow him to go up and smell each one. They need to smell and identify him as a member of the flock instead of as a predator.
Remember that an LGD is not a good obedience dog. They do not respond to training like other dogs and if they believe there is danger they will ignore you. They can hear and smell what we cannot so trusting your dog is imperative. However, in Leonidas' case, he has not received
any LGD training so trusting him will take a long period of you training him.
You are doing great. Hera is giving him a wide berth because last time she punished him she hurt her leg.