If it's arthritis there are a number of things I have tried that seem to help
Dasaquin: a chewy supplement that I get from Amazon
Adequan: a shot that is given that is supposed to help rebuild cartilage
Metacam: a liquid antiinflammatory that is given orally based on the animals weight
My lab Drake (10 this month) has an arthritic elbow and he does well with the Dasaquin and Adequan shots. The shots start out first 2 shots a week for 4 weeks, then after that you have a maintenance shot. He's been on it for a year now and initially he responded very well to the first 4 weeks. He's been getting a shot once a month but I think for him he needs it every three weeks and so that's what I'm going to begin doing because at least recently, you can definitely tell when he's ready for another shot. But it has helped a great deal.
Metacam has been an outstanding anti-inflammatory that both boys will get. Kenz (around 13 years old) gets it every night because he has bad hip arthritis. It seems to help a lot. Drake gets it pretty much after he's been working hard, usually right after we're done hunting.
Kenz did VERY well with Dasaquin but he was allergic to something in it. He was so miserable with itching while he was on it. Because of his allergy to the Dasaquin, I didn't even try him on the Adequan because the active ingredients are so similar. I didn't want to take the chance with him possibly being allergic to the shot. So be aware that there are possible allergy side effects.
Those three, all together or in some other combo or by themselves, is what I'd suggest if the bad hip is arthritis related. The Adequan and Metacam are prescription so you'd need to talk to your vet about it.
It sucks watching our dogs get old.....truly does......
Dani