I totally understand how you feel, as I have been in a similar situation, but with my biological daughter. She was reconciled with us after several years of estrangement, just before DH was diagnosed with terminal cancer and died 9 months later.
He had come to terms with the past and all was forgiven between them. He was a wonderful, loving man, who never held a grudge.
I don't know, but I assume that DD will regret, if she doesn't already, the missed time she could have had with him. She abruptly cut contact, got married, had a child, and moved house, all without telling us (although she sent cards on birthdays etc). She has never really explained it, although there were some issues with now SIL. But she says that she was 'very young'.
Now DD is being loving and thoughtful and is in contact every day. She just wants to go back to where we were , both with me and her sisters. And on the surface we have, but underneath I still feel angry and hurt, both for myself and for DH, who was deprived of the first 14 months of DGD1's life. So I would feel exactly the same as you do.
However, we can't change the past, so we have to come to terms with it. Your DH wants reconciliation, so you have to support him this and be pleased for him. Try to let go of the angry feelings, for your own sake and DH's sake rather than for the sake of your stepson.
I wish you well