Adoption

So my husband now has raised my youngest son since he was a year old, he's 3 now, he calls him daddy and has done everything a father could for him. My sons BIO dad hasn't seen him in a year and a half. But would ask about him every couple months. Well recently I haven't heard from him. About anything. He did move out of state and hasn't asked about my son in almost 6 months. My husband wants to adopt him. But I don't know what to do to get it started. Does anyone have any advice?