Legs problems with my puppy

 Hi

I have a lovely german shepherd puppy thats 4 months old. He's having problems with his back legs. He doesn't stand up straight, he seems to favor the bottom half of his legs rather than his feet. He also drags his feet when he walks. Thinking back he's been like this since i've had him i just didn't fully realise and its more obvoius now he's bigger. I didn't think he was in pain but now that he's finished his shots and can meet other dogs i think he might be. He's timid of other dogs and is always trying to protect his back end. If the other dogs get to excited he yelps and tries to run away with his back as low as he can get it.

I've been to the vets and they x-rayed him and said that his hips and joints look fine, his lower spine also looked fine. There not sure wots wrong but think its either nerve or ligament damage. He had trouble at birth and got stuck on the way out, the breeder had to help him out and my (uneducated) guess is that something happened then to cause the problem. 

Is there anything i can do to help the little fellow.  He's such a great dog i want him to be able to run around and play like i know he wants to. Any tips or suggestions would a great help.

Thanks Matt