Exactly this. You can't pretend that he doesn't have a dad. Of course he has a dad. Everyone how is alive has a dad or father, makes no difference what word you call it.
You explain it that it's not his fault that his dad doesn't want to see him but he chose to live another life without you or his son in it. Just be honest.
People make choices. They aren't always the right choices but we have to live with them and then it's up to us to live the best life we can, regardless of the choices other people have made whether they include us in their life or not.
You can tell your DS also at some point when he's a bit older, like 8 or 9 that he does have siblings somewhere but again, you don't know where they are or who they are because you haven't been given the information. You say it's a lot to take in but I have to be honest with you because it's important you know and I don't want there to be any information I have withheld from you. It's important you know about who you are and where you come from. Every child has a right to know that.
Then you say but it's fine, we're fine, you're fine, it's a bit of our lives that we can talk about any time though it's not something we have any control over, and that's sad and it's OK to grieve and be sad about it. It is a loss, a form of loss and a child will be sad that the person who gave them life, doesn't want to be part of their life. That's not how we expect to grow up.
So talk to your child. Be honest. Don't create secrets. At some point he'll ask, "where's my dad" and you'll have to be honest that his dad exists but at the moment he's asked for no contact with either of you and then hold your child and love him and tell him again and again, it's not his fault and that you love him unconditionally. And allow him to begin to process the loss. Because it is a loss and you can't expect him not to be sad about it. Of course he will be sad.