I’ve only just caught up on this thread
My first thought is that a dog holding a front paw is sometimes an indication of a chest injury (ribs or muscle) and may well be to do with him discovering the stairs
Exhibiting pain at his rear end, as you’ve already pointed out could be one of many causes. If it was HD I’m pretty sure you would see it more frequently given the apparent severity - he would also be a very grumpy boy (lip / nose licking, arched back etc) - my first dog had very severe HD.
I would suggest finding a chiropractor and having a chat with them. It is possible he’s caught a nerve in his spine that would give the symptoms you describe (Coral had an occasional limp and cramp in one rear leg which we are pretty sure was a trapped nerve - she had X-rays and in manipulating her hip joints it seems to have freed up her back/movement). At the very least they will be able to tell, from feeling his hip joints if they think this is the cause.
Hope it’s nothing more than a trapped nerve
Jayne
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