Ima, I can tell you my story which is similar but very diferrent to give you a view of the outcome long term.
My XP, who I thought was a wonderful man lived with me and my DS for 5 years.
He also had a DS from a previous relationship and because he had 50/50 custody and I was a SAHM I was one of DSC's primary caregivers and also loved him very much.
Our two kids started living together when DSC was 3 and DS was 5 and they grew up only really knowing a life together as a family of four. The kids were more than real brothers - they were best friends and pined for each other when DSC was with his birth mother.
I dealt with scraped knees, and bad dreams and told stories and bought school uniforms and went to school plays and as far as I was concerned DSC was my child too.
My XP, out of absolutely nowhere, decided he did not love me anymore and left on a Thursday when DSC was at school. Unlike your marriage he had never told me he was unhappy, so I had no idea it was coming.
He'd organised behind my back his own apartment and he left with just a short note to say he was gone.
He never allowed the two children to see each other since that day 14 months ago. He never allowed them to say goodbye. He never allowed me to see or say goodbye to DSC.
It was just a case of him being the most selfish, heartless bastard alive and I genuinely never knew it.
In terms of outcome, more than a year on my own DS who is now 11 is still very sad for the loss of his brother. He doesn't miss his stepdad, but he still cries sometimes for his brother. He can't play certain games, he can't play with certain toys, he can't go to certain place and in many ways what XP chose to do hurt him immensely.
That said, children ARE somehow resilient and tend to live in the moment and as long as his life is otherwise happy he manages to get on with it. I try and talk to him as much as I can, but of course I hope and pray there is no long term issues.
DSC was the younger and more sensitive of the two and I heard from people that he was crying a lot. It must have been very traumatic to come home from school to find he lived somewhere else and his stepmum and brother were gone and never coming back. My XP told him that WE left, not the other way around.
It broke my heart, no question about it, and life has no explanation for the evil of other people but I take some comfort knowing that our kids loved each other, will continue to love each other, were raised as brothers and will always be so. Nothing my prick of an XP can do will ever cause them to forget each other and in a few years time they will find each other on Facebook and my XP can answer to the truth.
I am sorry your H is saying these things to you and making these threats. I hope they are only threats, but sadly in my case it was followed through. Can you try and contact the birth mother to make arrangements for the kids to see each other? you might have no legal rights to see him, but social pressure can be provided.