My 4 week old calf has his front knee that is swollen to the size of a baseball and he is limping what is the treatment for this or should he be put down?

Either he has joint ill, or he has a bad abcess on his knee. Either one should see a vet ASAP. It's not worth putting him down if it's something worth fixing. He is a baby yet, and baby calves tend to heal up faster than adult cattle do.