I understand your pain. I adopted a foster child who came to live with me when she was 7. Her caseworker warned me that no matter what kind of relationship my adopted daughter and I had, her birth mother would always be an important part of her life; that she might not ever call me mom, etc.
She did start calling me Mom after awhile but she longed for her birth mother's love. I frequently felt like an outsider until I learned and accepted that this wasn't something personal. Babies do form an attachment to their birth mother before they're ever born and typically this attachment lasts no matter how the birth mother treats the baby/child. I suspect that this is caused by the drive to survive.
My adopted daughter always loved me. She wasn't able to tell me so until she was an adult. She also didn't tell her birth mother she loved her nor did she talk about love in connection with her. Her birth mother's parental rights were terminated and whether or not she saw her birth daughter depended on me. I had to prevent visits because she was destructive to my adopted daughter's well being.
When my daughter was an adult she resumed visits and would often choose being with her birth mother instead of me. She defended her birth mother and criticized me, etc. Along with those actions she still called me Mom and did rely on me, and not on her birth mother, for emotional and physical support. I felt hurt and very confused.
My daughter is now 30 and no longer speaks to her birth mother. She's apologized to me for her insensitivity to my feelings. We've had long talks and I understand that she was working thru her own pain and was unable to consider mine. I knew that at the time it was happening but it didn't stop me from feeling hurt and sad. I grieved.
At the same time we were both in pain, we also shared many good times mostly because I did not back off. I kept including her in my life. If I'd been you I'd have put myself on the phone to talk with your step-son. I'd keep contact with him thru phone calls, e-mails, and Facebook. I'd be sure that he knows that you love and miss him.
He's so young and feeling confused. As you hinted, he may feel that if he pays attention to you his birth mother will reject him. He may feel that he has to choose between the two of you. You didn't say what her attitude is. He lives with her and may have to be really low key in his involvement with you. He may feel that you don't want to have as much to do with him as you actually do because he chose to live with his birth mother.
I strongly urge you to be the one that keeps contact with him, keeping in mind that he's not far chronologically from being a teen and in fact his brain is still not fully developed. He isn't yet able to put life together in a mature way. And.....he is most likely handicapped by being abandoned by his birth mother at the beginning. He has much to work thru emotionally.
So do you, but you have more life experiences and a mature brain to help you. Counseling may give you a boost in a less painful direction. It's likely that his distance is not only causing you pain, in its self, but is also triggering past painful feelings from the past.
I believe that he loves you but doesn't know how to show you that love. Find ways to make it easier for both of you to be in touch. Perhaps you could even plan to get together in person and to talk on the phone so that you can tell him how much you love him and how much you miss him. At the same time let him know that you'd like to understand his situation and how he feels.
Please also know that people often treat the person that they're less sure of better than the one with whom they share love because they know that the person who loves them will love them no matter what. My daughter was trying to win her birth mother's love. It didn't matter that she hadn't taken care of her when she was a baby and child. If she could only do or say the right thing, her birth mother would love and care for her. It was when her birth mother put my daughter's children in danger that she stopped talking with her and seeing her.
A side note. When he was a teen he was acting very normal when he didn't have time for you. One has to insist that their teen spend time with them. Please insist now when you visit where he lives. Don't accept no for an answer. Because you didn't find a way to see him he may feel that you don't care all that much. Be honest about the way you feel. Be confident in your right to spend time with him. Make it happen!