Help me adopt my step son

I have been in my step sons life for the last 5 years. And from Aug 2014-March 2016 his mother was not in his life. Then she saw him 22 times from March 2016- Nov 2016. It has now been 20 months since she has seen him. 
He has had enough to deal with, and her being in and out of his life is doing nothing but messing with him. I love him more than anything. No different than I love my own. All I want to do is adopt him, so he never has to be pulled back and forth or left again. Lawyers are expensive and I want to do this right the first time. He deserves that. I debated asking this. But this is important to me and to him.