Thank you for your question. There are a few things that could be causing the signs your are describing.

• Sprained muscle: probably the least serious problem, but also potentially the hardest to diagnose. I would assume this was the problem if nothing else could be found
  
• Luxating patella (dislocating kneecap): this is a common problem in young small breed dogs. The patella can slip in and out of its proper location and can cause quite a bit of discomfort. Given the type of dog involved, the history, and the explanation of his clinical signs, I would put this problem high on my list. Depending on the severity of disease, surgery may be the best treatment option. Pain medication can be used to provide more comfort, but without surgery some of these dogs can develop severe arthritis. Surgery doesn't guarantee no arthritis, but dogs that have surgery have significantly less arthritis.
  
• Ruptured cranial cruciate ligament: same as a ruptured ACL (anterior cruciate ligament) in football players. Not as likely given the whole clinical picture, but still possible.
  
• Dislocated hip: this can be intermittent and may need to be surgically repaired. A sling and strict cage rest can be beneficial
  
• Fracture: doesn't seem as likely. I would expect more pain when you mess with his leg, but this is still possible. X-rays of the paw, leg and hip may reveal that this is the problem.