HI Alexis. This lameness in your border collie is most likley due to severe hip arthritis or arthritis in his back vertebrae. This can come on suddenly by your guy moving in a certain way to irritate this area and pinch off the nerve supply causing pain and discomfort. For now, he needs an antiinflammatory agent to decrease the inflammation in this area affected and release the nerve supply. I would also suggest you add a supplement of glucosamine and chondroiten to help lubricate his tired joints. This can be purchased at any pet store. The antiinflammatory agent is aspirin at 5 mg per pound of his weight given twice per day.
Monday, have your vet examine him and possibly take radiographs to determine where the arthritis is located and the extent of the problem.
Dr. Loretta I hope this helps!! Let me know if I can be of further assistance. Click REPLY if you need more info. Click ACCEPT if I have answered your question.