What do I say?

 What do you say? 

I think kids usually only ASK as much as they are ready to "know".   So, what did Kale say?  The actual words?  Answer THOSE words.  No lies, no extras and as tactfully as you can. 

For example, you might say, "OK, so what did you want to know about your real dad?"  He might say, "Where is he?"  Your answer might be, "Honey, I don't know."  Let him know by your body language that you are engaged in the conversation, so that he is not put off from asking a further question if he wants to.  Let him "lead" the conversation.  He might ask what he looked like.  Or he might ask something trickier like, "why didnt he want me".  And the answer to THAT is probably, again, you don't know.  You can't imagine why ANYONE "wouldn't want" Kale.  You're proud of him, he's a great son.

He might ask more detailed questions about the circumstances later... and IMO you are better off answering the questions as they arise, not sitting him down and talking to him about it all in one fail swoop. 

 Thanks Chuffy.  Kale's main concern as it turns out, is whether he "Gets" anything from his dad.  Mainly, his love of Pokemon.  LOL.  I'm sure many more questions will come up as time goes on, and I will deal with them as they happen.  At least now the ice is broken and he's free to ask when and what he wants.  I made it very clear to him that I would always tell him the truth and that I would answer anything I possible could.