It just takes time and consistency to achieve this. One of the best training exercises you can master and teach your GSD is to focus on you. Using a 'look' or 'watching' word, issue the command, at the same time use a high value treat such as hotdog meat or cooked chicken, wave it in front of his nose, bring the treat up to you eyes, the second he looks at you, mark it by saying 'yes' or 'good boy' and reward him with the treat. Practice this at home 10 times a day and everytime you are out walking. Eventually you will be able to get him to look at you for long periods.
When you are walking and approaching another leashed dog, do your focus command, pick up your pace - keep moving forward, even if he lunges at the other dog, just keep moving forward and command him to focus.
Also, I don't know how old he is but he looks like a juvenile and imo no longer needs to meet and greet other dogs, don't allow him the opportunity to meet strange dogs, train him in obedience and keep his mind and body active.
if your GSD is eating and eating and eating and losing weight - please consider testing for EPI.
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