I walked my dog today and in the middle of the walk, my dog's hind legs would collapse under him. I thought he was tired so I just let him stand there for a little while, before I started walking him again. He started doing this 10 minutes into the walk, then a couple more times along the way. He wasn't yelping or showing any signs of 

ain that I could see when the hind legs collapsed under him.
The total walk lasted about 25 minutes, and it wasn't a brisk walk or a power walk or anything like that. It was just a normal slow paced walk.
Why do you think this happened to him, is it the arthritis that he has or is it that his muscles in his hind legs were getting tired? And what diet, exercise should I give him to improve his condition? He's an 11 yeard old German Shepherd that hardly gets exercise. I started excercising him today when I saw him do this.