How likely is a cracked patella?

The patella is a pretty tough bone. If the doctor did not tell you that the patella was cracked, then the patella is probably not cracked. The x-ray would have shown a crack and the doctor should have told you if that was the case. He probably also would have fitted your son with a knee brace. Keep using the ice for the swelling. If your son has trouble walking, you may want to invest in some type of Ace bandage or knee brace until he feels better.