My husband wants me to adopt his son?

me and my husband were married for 4 months. i have 2, almost 4 children, and he has 1, almost 3 children. ( we are having twins soon ) My stepson is very close with me and we share a good bond. He spends alot of time with me and we do lots of fun stuff with me, him, and my daughters. His mother move away and she was very immature, but still young ( 21). My husband said he wants me to adopt his son. This all just came out of no where. I was surprised, but I would love to adopt him. Which steps do I take? Thanks.