Stop trying to walk him. He is in pain if he is only using three legs.
First thing that comes to mind that he could have a torn CCL (in the knee joint), very common in the breed. It takes surgery to repair it.
Second thing that comes to mind is osteocarcinoma (bone cancer)....very common in the breed also.
There have been a few threads started just this week with dogs being diagnosed with it.
Please get your dog to a vet...and have him checked out.
Gina