Michelle, as your farrier has stated, it can be caused by a lack of exercise, poor or no farrier care, and perhaps too much weight. Sometimes, just having the space to move around and the inspiration to move is enough to build up his knee strength.
This is a different problem then those that are born with an obvious genetic issue.
Proper exercise, farrier care, and nutrition are the first course of action. I would go with your farrier's advise for now. It can take six to eight weeks to improve or more. Free moving is also important.
Best of luck to you.