Thanks for all of the advice so far.
We decided to take my horse to a University Veterinary Hospital to have a diagnostic evaluation.
So far the investigations haven't yielded any results.
What we know so far is that the lameness is in the right foreleg.
After blocking the right foot successfully the vets determined despite the odds the pain is not originating from the foot. So that rules out sidebone causing the lameness. I was told that they were 99% certain they blocked properly and that the foot was not involved.
Next the vets evaluated the fetlock and ruled out that as a possibility. Again they were confident with their nerve block that the right fetlock is not causing the problem.
Next came the right knee. Regrettably we ran out of time as the clinic was closing and we couldn't leave my horse overnight for further investigations (hadn't packed for it). We had to turn around and come home.
We were told that 70% of lameness is in the foot. And that it's possible that the knee is involved. The approach should we proceed would be to progressively block the nerves innervating the knee and do more xrays of the knee and possibly do an ultrasound for his shoulder...We already have xrays of his knees (only 2) and are considering different angles.
I am really disappointed and frustrated because I want to know what's going on to cause him pain and what I can do about it. I am also frustrated because I think in a 16 year old horse who doesn't have a history of acute trauma and with no history of swelling or inflammation there must be a limited number of things it can be.... Also please keep in mind he hasn't improved AT ALL over his 4 1/2 months of rest. Initially we would "work out" of his lameness after warming up. In December he no longer 'worked out it" (He was only ridden for 15-20mins 3 times over the 4 months).
Here is a video of him in walk. BWJ walk 1 - YouTube
Additionally, he had no relief after being on painkillers for 20 days. He was still lame.
I understand shoulder injuries are really unlikely, but how do I rule this out? What about likely causes of injury to the knee or above?
I am really devastated... I've had my horse for 10 years and after eventing at Entry level when he was 6 years old he has only been schooled in dressage. He is not worked too hard. Any advice you have would be great...