Hi. My name is B.. I had a "step" daughter whom I raised since she was 4 also. She passed away from cancer in 2005 when she was 10 years old. I have three children of my own with her father.
I have alot of advice on this issue, some you may not understand, some it's just too much to type, lol. I was a stepchild. My step mother treated me like literal dog sh*t. I moved out of my dad's house when I was 14 because of her. I promised myself that if I EVER had a step child that I would love them just as my own. I made good on that promise.
My first piece of advice to you is don't refer to him as your "step" son. You didn't step on anyone and neither did he. If you aren't comfortable with saying you are his mother, then say you are his "other" mother. That right there, when you put that "step" in front of "son" picks him apart from your other children and causes him to be on the outscurts. If you want to treat him as your own, the first step is to call him your own. Especially in your case where he knows you are all he knows as a mother, you have a special bond there with him.
It's hard for me to sit here and try to tell you what to do. I can't change the way you feel, I can only tell you what has helped me and hope that in some way you may feel the same at some point in your life.
When I looked at my "step" daughter, I didn't look at her as someone else's child. I looked at her as if I were her mother. I looked at her just as if she grew inside of me and I was the one who gave birth to her.
My husband still to this day tells me that I treated Brit better than my own children sometimes. That's because I didn't want her to feel the way that my step mother made me feel.
I hope in some way that this has helped you. Please don't hesitate to ask me anything else if you need to. Good luck! And I applaud you for trying to be the mother that your son needs.
B.