Welcome. I'm Dr. Bruce and I've been a small animal veterinarian for over 15 years. Thank you for your question. I'm sorry to hear about Dingo's situation. With what you're describing, arthritis
/ hip dysplasia would be a very high suspicion. The medication that he's on, prednisone is a steroid and it can help with some discomfort associated with arthritis. I'll be honest and say that I like to use an NSAID like Rimadyl
for arthritis management as it has less steroid adverse effects. Here is a good link on arthritis management.
Here is a good link on hip dysplasia.
Could this be something else? The most common other cause of a single rear leg issue is a cruciate ligament injury - a partial or full tear. Here is a link on it.
Depending on what is going on, that will dictate what the end result is. With all of these, he won't necessarily get back to normal but hopefully be controlled with his discomfort. If it is the cruciate ligament injury, surgery could help to get him to the best mobility and comfort level.
You could always get him rechecked and see if the vet is still thinking along the same line of thought.