You could try Ibuprofen, or half, depending on his weight. I don't think I would give him anything myself, dogs have a way of sucking up pain, they aren't wussy like us. He's made it this far without anything so why not wait till you see your vet tomorrow...you never know what it can be so it might be better to wait it out. I'm sure if the pain was out of control he would refuse to put weight on the leg , and usually their appetie diminishes with severe pain.If he's eating and drinking well then don't worry so much.