Sore hocks are painful for rabbits. If its bad enough hes refusing to stand properly, he is in pain. Rabbits are prey animals which means theyll never show the amount of pain they are in; in the wild, prey animals who show signs of pain are the quickest to be killed. Remember, sore hocks bad enough where the rabbit cannot walk, stand, or sit properly also means the hips and spine are being affected. Arthritis (constant pain) and dislocations (which can lead to needing to put your rabbit down) are a likely (near; age is only a small factor) future if the sore hocks are not cared for ASAP.
When you rehome him (because you should; this bun is suffering), tell the shelter everything the vet told you about him. It would be even better if you could give the shelter your vet’s name and number so they can get the direct information from the vet. This way, his health wont be a set back, itll be a leap forward in the right direction.