Here you can see him picking up his leg:
So it turns out what is happening is his knee keeps popping in and out of place. The vet says this is pretty common in small dog breeds where their knee joint is to shallow. Basically, they have to go in and make it bigger so his knee can fit on top of it better. This should fix the problem, but he could develop arthritis there later in life. So a week from tomorrow Mister goes under the knife. The surgery should only take an hour. Of course, I'll update you right here.
In the meantime, I'm carrying him up and down the stairs, limiting his walking, feeding him anti-inflammatory meds, and keeping him from playing with the Missaroo. None of which is an easy task. Poor Limpy!