Luxating patella is what came to mind here. Both of my pups have them, and Myndi's is the worst. She can't jump up on the bed or furniture, or go up stairs anymore, and I have to make her settle down if she gets a little too rambunctious, or it will go out on her again. She absolutely screams when it does, so I try to keep her out of situations that would tend to pop it out of the joint. Both of them have managed quite well tho, and they both get glucosamin/chrondritan (sp??) daily, and Myndi a baby aspirin if she appears to be somewhat uncomfortable.

Hoping you find out soon, what is ailing Henry. Sending good vibes and positive thoughts!