The couple became parents on Christmas Day, 2010 to Zachary, who was conceived using a donor egg and born via a surrogate.
David said of his young son: "I spent this summer teaching him to swim."
He added that when it came to expanding their family: "We are looking at it but it's not just a matter of clicking your fingers and it's done. It needs a lot of planning and thought."
David, who was speaking at the GQ Men Of The Year awards, said Elton was loving his new role as dad.
"Elton sits him on his knee and plays the piano for him. In fact I just spoke to him and they were together, Elton was playing the horn and Zachary had the triangle but he's sporty more than anything," he said.