Adopting my stepson

My DH and I have not been married for that long but we have been together for a few years. We have 1 beautiful boy together and I am expecting another baby boy in Nov. My hubby has a 14 year old son with his first wife. She left my DH over 10 yrs ago and remarried and had 5 children. She has not made any physical contact with my DH or my stepson since she left. To my knowledge she has only called her son once or twice and the last time was more than 8 years ago. She has never paid child support. Before me, my hubby remarried his 2nd wife. They were married for 6 yrs. When she left him there were 3 children involved. (Her bio-son, his son, and a daughter they had together.) They did not want to split up the children so his son lived with his 2nd ex-wife even tho she was not the Bmom. They are amicable so they did their own divorce lawyer free and she was given custody of all the children. Two months ago she called us and said that his son had to come live with us bc she could not handle him with his behavioral problems so he now lives with us. I love my stepson as if her were my own child and I want to adopt him. I think part of his behavioral problems are from not having a stable home and from his bmom abandoning him. Do my DH and I have to be married for a certain period of time before I can adopt him? The bmom has been MIA for years now... is it easier to petition for TPR or to try to find her and gain consent? I'm nervous (I think bc I know so little about the subject). This is something I never thought I would be doing, but once my husband and I discussed it with our son I knew in my heart it was the right thing for our family. Any words of encouragement are appreciated.