My chihuahua loses all leg control and falls over right before eating and treats. Is this a brain disorder?

5 years old, manifest in the past week, previous to that would briefly whimper before eating. Eats all of his food, clear and regular stools, seems weak. Saw vet a week ago before falling symptoms, x-rays returned negative for tumors, fractures, etc. Vet associated pain before eating and weakness to arthritis in the hind knees (Luxated patellas). He can stand and walk fine until you put a treat in front of him, and then his legs paralyze and he plops to the ground. Any help would be awesome as we love this little guy!