umadevi, you don't know me but I can tell you some things that may help you work on this with your son.
"Z" was right, his viewing of videos, tv Utube or association with other children (who have older siblings) can cause him to feel that his action of pulling the knife being normal. This was VERY serious and regardless of his excuse, needs some deep review. And yes, coming from a Single Parent family, is a serious issue for many children.
I would suggest you see a child therapist to help you and him sort through this - you can't afford a repeat, of this - no one can. He does ahve some anger issues and at 9 YO this needs attention. Please remember there are reasons for EVERY action (Cause and effect) that a young person does,
I have raised 2 children myself- they are mid 20's now and I can tell you sometimes we need to reach out for help before it escalates.
I am a product of a broken home, and his disappointment with his father will only get worse, he need a Big Brother (they are available) to help him with his self respect and make him feel he is a growing up to eventually become a man. I can't stress this more than from 8-12 are the most formative years for a boy that will help decide the type of character he will be. Find him him somebody that will take a genuine interest and teach him to become a man. His requirements are very different than a young girl.
Also his reaction to "playing fair" leads me to believe he has a low tolerance for mistakes and others, the world is not fair, and he has to realize this, if he gets put off by cheaters at 9 YO, he will have worse problems at Middle School and High School when MOST children will never be fair.
This is my advice only, I by no means am an expert, just some things to think about,
We all wish you the best in dealing with this heartbraking situation
DG