I haven't read this all, apologies......
I exploded and called my teenage son a cunt not that long ago. So, I apologised, explained that it was not ok no matter the reason for me to call him or shout that directly to anyone I care about.
I'm the adult - as is your husband. He really should own that outburst and take accountability for that.
However, I'm now clearly seeing that my teenager, although with additional needs being Autistic, has consistently and worseningly behaved in a way with me and towards me with increasing severity that's tantamount to abusive. And I have soul searched endlessly over this. My outburst was inevitable, that's not an excuse , but many would have lost it years before I did.
The resolution - well teenage son can't see any issues with behaviour towards me and I believe this will develop and worsen into adulthood. Contact is therefore restricted; chosen by me. There is no other answer than this based on my personal situation.
Because your partner is a man, it tends to be a different dynamic. Maybe he has been acting like an absolute cunt and you can't see it with your rose tinted mum spectacules ( something I still struggle with). Maybe you'll never over come biology. He's not his biological child and does that play a part, maybe.
I would speak to a counsellor in your shoes. I would want to see your husband acknowledge that it's not ideal for him to be saying cunt to your son and take some steps to try prevent that again.
Knowing I'm a loving and giving person who is not an abusive arsehole, but usually on the receiving end - and I shouted 'you horrible cunt' myself - well that makes me much more open to not automatically condemning your husband for that one act alone tbh.