Could you please tell me a bit more about what happened?
How did your son hurt the kitten?
Do you believe the injury was intentional?
What was going on coming up to the incident?
It sounds like it was probably accidental and that he was afraid he'd get in trouble. Judging from what you've told me about his past experience with the kitten, it sounds like he was generally really good with it. I think you need to speak with him, let him know that the kitten was hurt very badly and that in order for it to be taken care of properly, you really need to know what really happened. If he still does not want to tell you what happened, let him know that you know how much he loved the kitten, and how upset he must be that he hurt it, but that he can help by letting you know what happened to the little guy. Try to get him to talk about it, as he is probably feeling really bad about the entire thing. Even if it was intentional, it was probably done in the moment, but it will be important for you to figure out whether the injury was accidental and intentional. Either way, discuss what he could have done differently, and how responsible hewas for letting you know that he had hurt the kitten (he could have just left it there any hid or told you he didn't know what happened, it took real courage on his part to bring the kitten to you).