I am sorry your dog is having trouble. Applying a splint at home is not a good idea, especially if you don't know what is causing the swelling. Even though it might seen easy to splint a leg, it isn't and you could wind up making the situation much worse. I am worried about this swelling, because your dog is older. There might be an auto-immune component present that is causing inflammation, or a tick borne illness like lyme disease. Or, your dog might be experiencing a type of cancer that involves the bone, called osteosarcoma. To properly manage this swellilng, some x-rays will be necessary, and some blood tests might also be necessary before your dog can safely be put on pain medicines. You should use aspirin with caution since it can be harsh on the liver and kidneys, and these organs might not be functioning normally in an older dog. I feel that splinting the leg is definitely not the best thing to do here, and that your dog really would benefit from some x-rays. Please let me know if I can be of further assistance. If this has been helpful, please hit the green accept button. Best of luck with your dog, I hope nothing too serious is going on.