G.S.
I can only speak from my own experience. I'm the product of my parents' marriage. But they were each married once previously. So I have half-siblings on both sides. My dad's two children stayed with their mother. I only saw them occasionally, when she allowed them to visit. My mom's son came along with her, and I grew up with him as MY BROTHER. Interestingly, my dad did not "adopt" my brother. My brother lived with us (and occasionally visited his father), and he kept his last name. No one ever questioned this. My brother was quite young at the time that my parents got married (and then made me), about your son's age actually. We were by no means the perfect family. But we did just fine. There was never any question that my dad was my brother's dad (we never said step-dad, ever). I don't recall there ever being any tension between the two. (My brother and my mom, on the other hand, had their moments!) My dad and my brother have always had a good relationship, and my father has treated my brother like his own and has been very proud of my brother along the way. (We are both grown, now -- my brother in his mid-30's, happily married and father to his own young boy).
I should note, too, that my brother did NOT have a good relationship with his "Bio-Dad" (that's what he calls him). And he still does not. He was able to go stay with his Bio-dad in the summer sometimes as a kid. But his visits to his Bio-dad got to be less and less frequent. And I think his Bio-dad just lost interest in him or something, and my brother stopped visiting him altogether. And I *KNOW* he was unhappy about it. I know he felt rejected by his Bio-Dad. I know he STILL feels some of that. But the fact that he had a loving and supportive FATHER at home was enough. I'm pretty sure my brother is not so emotionally scarred from his Bio-Dad's lack of interest that he's carrying around any significant emotional burden.
I think the thing you have to remember is that teenagers are crazy. Do you remember being a teen? I do. I do NOT look forward to going through that with my own! There's no predicting in advance how a child will behave in adolescence. I think all you can do is your very best as a parent while they grow up -- and hope that your best has been successful when they hit those crazy years.