Calling stepdad- "daddy".....

My son just walked up to me and said "I want (insert my SO's name) to be my real daddy and I want to call him dad!" He's 5 and my SO has been in our lives since my son was 18 months old. I've never mentioned anything about my son calling him dad, he's always called him by name. Out of nowhere today- he says that. It was pretty shocking. I told him "You can call him whatever YOU choose to as long as you know (insert fathers name here) is your real father. His father IS in his life, he gets him when he's suppose to. He works in the oil field so it's hard to see him often, but he makes it a point to see him when he's in town. My SO just has more of that bond with him since he's with us the majority of the time.