different adoption question

Danyel
Ok so I was 3 months old when my biological dad died... My mom remarried to my dad when I was about 2 or so and since then they for divorced she I was like 9 but he has raised me as his own ever since but I was never adopted by him legally I wanted to know How to go about surprising him with adoption papers on Christmas or his birthday! Does anyone know how to do that?!