First of all, i just wanted to say that I know where you all are coming from in this. I am 25, and I was 14 when I found out that my "dad" adopted me. My mom was young (19) when she had me, and my real dad was even younger (17-18). My mom thought it was better if she lived with her parents, than to make a boy (basically) give up his youth for a baby. Now, I know that doesn't sound like making him own up to his responsibility, but the fact is, her parents took better care of us than he EVER could have, so, it worked out for the better. My dad adopted me when I was 2/3, I think, and I have thought of him as my Dad ever since. He has always been there for anything, and I KNEW that he loved me, even if he didn't always say it.
Now, I found out a little by accident - I was flipping thru a photo book, and noticed that there was a boy in there who had red hair and green eyes. My eyes were just beginning to turn green then (it's happened to 3 of my friends, too - color change later in life), so I pointed it out to my mom. She then told me that the photo was of my real dad. I was a little surprised, but she explained the circumstances, and told me that if I had questions, to ask them. Eventually, I did have a few, and things got cleared up for me. Now, I am married to a wonderful man, and we have 3 kids and 1 more on the way, and they know that their Grandpa loves them LOTS!
I think that your son realizes who his "real" dad is - the man who has been there for him thru everything - and doesn't really mind that. The brother may have just added some stress for him, made him feel guilty for NOT blaming you, you know? So then, he started to think that he was SUPPOSED to be mad at you for not telling him sooner or whatever. But all in all, I think he knows that you know what's best for him, that you love him, and that the dad he has now is his Dad. I hope that's not confusing
But I don't think you should worry too much - if he is ever in need of answers, he WILL come to you.
Hope that helped a little! Thanks for letting me ramble!