Divorce him before he has his friend rape you in front of him. He’s a scum bag and in no way was this just a fantasy but a sick need for him to realize (nothing against those who do hot wife but this isn’t that obviously). There’s no saving this as he severely broke your trust and won’t accept your no.

Take you and your child out of this situation ASAP. Talk to your friends and family for support and take this asshole to the cleaners. He can have his fantasy with someone else who's willing to play along.