Hind leg muscle atrophy

[mclayton]

I have a 10 year old lab who has significant muscle loss in his left hind leg over the hip joint. When I noticed I took him to my vet which is affiliated with a shelter and has done his yearly wellness checks. They don't have xrays because it is low cost and finances are a concern. The vet checked him out neurologically and did not notice anything that could be detected on physical exam. She said it might be arthritis and so he was started on glucosomine & chondroitin. He has never appeared to be in any pain. He is happy and playful. Jumps up on the couch and bed but you can tell that his hind leg lags and is weak. The cold I noticed seems to really affect him. When he goes out and maybe when he has a bowel movement he really struggles to hop up one little step. I also found him sitting in the snow like he wasn't able to move but when he saw me he did get up and walked in. Once inside he seemed fine again. He even snuck into the basement were he is not allowed. I was scared when I went after him because I thought he would not make it up the stairs because there are 14-16. I went behind him and it hopped up without any problems. I'm worried I should be doing something else for him but I don't know what. I can't afford thousands of dollars for tests just to be told care is supportive. Does anyone have any suggestions about what this might be?
Other than the few episodes when he is outside nothing seems to be bothering him, it bothers me. Also, I don't leave him outside for long it is just to do his business and then I get him right back in, that's why when he did'nt come back right away I went looking for him. Any thought would be appreciated.