I would take him at this point. He is in pain, otherwise he would not be holding it up.
Last Memorial Day, Beau was being silly in the yard while I was out there and I heard THAT scream and he came tripoding around the pool screaming with his leg up. I immediately took him to the vet and he had pulled the achielles heel away from the bone and needed 3 hours of orthopedic surgery. The vet said, in 35 years, he had never seen anything like this....
So... you never know what you will find with x-rays. I hope it's just a pulled muscle. but I would not wait any longer.