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I wish you the best as you go into this with him.

How about family photos/photo albums?

You could look at photos of different family members (grandparents, you, etc.) and use that as a lead in to the discussion of ancestry, birth, family, etc. Don't make it over-emotional, just age-appropriate facts. Do your best to try to frame it for him so he understands why he's being told this and what it means for him now. He may have several different reactions. The key to remember is, let him go through whatever emotional processing without taking any of it personally or getting defensive. Remember, this is his story and his life and he's going to have questions about his own story, whether immediately or later.

This will be a lifelong conversation- not a one-time "whew, that's over." Start the conversation now, and be prepared to continue it for the rest of your life.

Best wishes!