I have to say that I feel terribly unqualified to answer this - but I find it a compelling question.
I don't have an answer, but it reminds me of how I feel about my Daddy, who passed away in 2004. I hate that Sloane will never know him, but I want her to have a sense of who he is. So I have a picture of him, in his prime (skinny with lots of thick, dark hair) framed in her nursery. It's not in a very prominent place, but it's still there.
And I talk to her about him in way that makes him seem "real" - not like he's going to pop in for a visit, but in a way that lets her know that although she'll never meet him, he matters because he had a significant role in shaping who we are as a family. I think the biggest thing that will help her understand who he is/was is letting her see and feel my love and appreciation for him.
I know your situation is very different than mine, but I believe Mikey will learn so much just from his perception of your feelings toward his birth family. The fact that you continue to send cards and pictures says a lot about your committment to his birthfamily - and to him. I hope someday they choose to establish some sort of contact and tangible interest in him.