"Oh daddy," the kid said. "I love you so much!"
"Hey," the man responded. "Until we get the DNA test results, I'm just Harry to you!"
The humor comes from the fact that the man is responding to a child who calls him "daddy" with a statement implying that he might not actually be the child's father. He's waiting for a DNA test to confirm paternity before accepting the title of "dad". It's a dark take on the usual family dynamic.