"The first thing I do every morning is have breakfast with him. He shares my breakfast. Whatever I have, he wants.
"He has my boiled egg and I have his pineapple or vice versa. I sound like I'm the only person who has ever had a kid - everyone's been through this.
"But for me, it's an incredible difference to what life was before.'
The couple "definitely" want to add to their family but Elton explained, "at what stage we don't know, but we definitely will. I don't want him to be an only child".
Zachary also appears to have picked up some musical tips from his famous father.
Elton explained: "He loves the piano, he sits down and plays it - he doesn't just bang it, he plays it like I play it. He's never seen a show, he's too young, but he comes out when the stadium is empty and sits down on the piano stool with me and he plays and looks into the microphone and thinks, 'What the hell is going on here?'"