What breed of dog do you have?
How long has he not been putting pressure on it?
Was he limping off and on before he started keeping it up?
Do you know of anything that may have caused it?
Have you checked inbetween the pads and the nails for foreign objects?
Thank you for that information. Larger breeds are prone to torn cruciate ligaments which would show intermittent limping to total non weight bearing lameness. You may see it over time or it may happen suddenly. Your vet will put the dog's leg through a range of motion while feeling the knee, as well as watching how the dog walks. IF it is a ligament then surgery will be likely. I am going to direct you to a site on this that will tell you symptoms and treatments.