He really has gotten worse. It's terrible!
Okay I'm sorry, I haven't even read all of the replies here, but this is my first time back on in a few days, and I needed to say this. PLEASE don't think I'm crazy, but I had a dream about Yukon the other night. In my dream I was not the vet or the assistant but just there. The vet was doing an exploritory surgery of his shoulder as opposed to an MRI, and found a small bur on the part that links the shoulder bone to the leg bone, I don't even know what they're called I'm sorry, but in my dream I saw it, and it was just Teeny Tiny, but definately there, and attached to the shoulder socket thing. I don even know if this can help you, personally I'd probably think I was crazy, and I have no IDEA what the anatomy of a dogs shoulder bone looks like. But...Maybe it's something that the vets haven't considered, a small burr or slight abnormality in the growth and formation of his shoulder.
Again, I'm sorry for sounding crazy. But I'm really hoping things get better soon.
Big hug for you and Yukon, and Yiska cause I'm sure she really misses playing with her brother.