Like AF, I grew up with this. You've had great advice regarding standing up for your son, and the possible effects on him, so I won't repeat it.
I want to suggest that I don't think that your son is the only person in your household experiencing emotional abuse.
Here's your exhange again:
Him: "It's just a figure of speech/only a joke. It's not exactly scarring him for life."
Me: "How do you know, getting stuff like that dripped into him all the time"
Him: "Don't be ridiculous, you're way over-sensitive"
etc etc etc. It's just I'm right/no, I'm right/no, I'm right ...
He cares more about being right than about your feelings or opinion. He cares more about being right than his son's feelings. My dad liked/s to call me oversensitive or defensive too. As discussed in another recent thread, he doesn't see other peoples feelings as real or important, he minimises or denies that his comments are hurtful.
Let me put it another way, so what if you are more sensitive than average on a global scale of sensitivity? Its you he's living with, not those others who might agree with him. Your feelings and your son's feelings matter. If one of you is hurt by something he said, then he should care about that, even if he didn't mean it. You'd care if you hurt him right? That's what people who love each other do.
So his response should be something like:
"Oh, you think that my words hurt our son? I just meant to lightheartedly tease him, but you don't think he understood I was joking? Gosh, I better go and apologise to him, thanks for letting me know."
Do you know what we call someone who continues to make jokes at the expense of others who find them hurtful in order to sound big/clever? We call them bullies. No one should have to live with a bully. Home and family are meant to be sanctuarys not battlefields.
how can I get through to him that it's SERIOUS, when he just plays it down? He genuinely, genuinely doesn't realise and I don't know how to make him.
I think that you make it clear that you will no longer put up with it. That you consider him bullying an eight year old boy who looks up to him appalling behaviour that makes you think less of him. The easiest way to show you are serious is to leave. But certainly every single time he does it in future you step in and let them both know you think its wrong.
Even if we split up he'd be forever thinking that I reacted way over the top.
I think you have to give up on the idea that he'll acknowledge you are right about this - if he knew you were, he wouldn't be doing it in the first place. It doesn't matter if he thinks you were OTT and tells his friends that, does it? As long as you've stood up for your boy. Your P is using your attachment to being seen as the 'reasonable' one to walk all over you. You don't need to prove to anybody that its reasonable to be pissed off with your son facing emotional attacks in his own home.
You care about your P's opinion more than he cares about yours: you want him to think you are reasonable, you worry he'll be angry if you film him. Is your partner spending much time worrying if he's being reasonable to you and his son, does he worry that you or his son will be angry with him when he's rude and abusive? Why not?