We rescued a 15month old GSD who had been in 2 homes. The 1st he was trained with fear and aggression,the 2nd one he started getting dominant and territorial. He was the perfect dog until he had to have a Cruciate Ligament op in 2019 then his 2nd one in 2020, both had slight complications which increased recovery time, ever since then his temperament has changed and he has started to get a bit aggressive towards us. Not all the time, so times he is very loving and playful but now and again you see that look in his eyes that he thinks you are going to hurt him especially being brushed or dried off after a walk. I think it stems from frustration and boredom due the lack if exercise while recuperating....any suggestions...??