I think it's a bloody good lesson in life to learn, that you can't demand people care about you, then do things that are harmful to you and not then let these people you want to care about you have an opinion and just worry endlessly.
But you aren't having an opinion. You have manipulated him in to not doing it by enforcing a hobby he doesn't want to do and saying you will do it too. I actually think you should do it. I don't think you realise that you will probably, in a decent club, get to the Point where you are in a ring getting hit in the head. If you do, it will be a long time.
You could have achieved the same, by showing him what happens to boxers. even though the injuries you speak of are really in certain categories. you have access to the internet, do research with him and TELL him you are scared and worried for him.
Your post about your ex doesn't change anything for me. The simple fact is that at 9 years old it will be a longtime before he has the choice of stepping in the ring and he may not want to do that anyway. When he is older he may choose to do it and there won't be much you can do about it.
I wouldn't want my son fighting and agree with you on that. But I also think you should learn more about it, for kids. Is it that you are worried he will try it, like it then choose to get in the ring and fight. What will you do when he is an age you can't manipulate him into not doing it? Will you still get in the ring, try and force him to take up ballet too. I honestly think educating him on the dangers would be best.
I would like ds to play rugby. DH doesn't, he thinks its too dangerous. But neither of us will manipulate him into making our decision.