Although Furnish was originally worried that their child would be "stigmatised" for having two gay fathers, the newspaper reports that he is now keen to have another child later this year.
He told the Evening Standard, "I would like Zachary to have a sibling. I think he would be better with a sibling than on his own. It's something we talk about all the time. But in terms of when, where and how, nothing's decided yet.
"I think we are going to wait 'til this summer. Our life is always like a treadmill with Elton constantly working and touring. But when we go away to our house in France, that's when we sit back and go, 'OK, how is this working, how are you feeling?'"
The couple is considering using the same surrogate who carried Zachary, and they will not know who the biological father is.
He said "We could go back to the same egg donor, so biologically there would certainly be a connection between brother and brother or brother and sister."