The pair welcomed baby Zachary at Christmas after a long process to find a surrogate mother to carry their child.
Furnish insists they will be completely honest with the youngster about how he came into the world, and they have been collecting mementos for the tot.
He tells Britain's OK! magazine, "We've taken advice from counsellors on this and we've already started a scrapbook with pictures of us meeting the surrogate, the first scans, letters and emails, hospital bracelets...
"We want him to understand how much love and care went into his birth. We've already got a couple of great children's books on how families come in many shapes and sizes."
Sir Elton also reveals they have chosen to stay in touch with the surrogate mum, who has chosen to remain out of the public eye, because they want Zachary to get to know her, too.
He adds, "We want the surrogate mum to be a part of our lives, and we want him to get to know her, because she's a great woman. He will always be told the truth."
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