I had a GSD who was a bit like this but in one leg.He had Rhumatory Arthritis in his hip.He was loosing muscle in that leg also.He was put on an all natural gell caps and shark cartlidge.
This was not a cure though...It did help for a while.He had to be carried up the stairs as he couldn't get up them.And it was no fun carrying a 100lb dog.This started when he was 9...He wasn't in any pain.Till one day we went for a walk and there was a huge yelp.He sat and didn't move.Well there was nothing that they could do.And I had to put my baby down.He was 13.
I'm sorry.I'm sure this is something you didn't want to hear.
Labs do have hip problems.
Has he had exrays done on his hips?
I'm sorry if I wasn't any help to you.