Appyt's advice is excellent, ESPECIALLY no stall time, 24/7 turnour.
Here's a simple diagnostic. Give him 2 grams of bute in the evening; the following morning, try picking his feet up and see if he's more cooperative. If so, you have your answer - it's not behavioral, it's pain.
Of course, you can't keep him on bute long term, but I would def give him bute the evening before and the morning of a farrier appointment. There's also lots you can do to manage arthritis in an older horse: MSM or other joint supplements, injections, mild, consistent forced exercise, 24/7 turnout.
If bute doesn't change his behavior; then you can feel free to address this as a training issue and assume he learned to get over on his handlers during his time as a therapuetic horse.
I'd bet that it has more to do with pain and stiffness.