"Kim likes to joke around with him a lot. She'll ask him if he's cheating on her, stuff like that. She knows Justin takes it more seriously but she would never do anything to embarrass him or make him feel stupid.
"She remembers what it was like to be 16 and have a crush on someone older. Justin tries to act like a 'man' when he texts or talks to her."
They added: "She's flattered and she pretends to take him seriously. At that photoshoot when he was holding her hand in the water, he was thinking he was the man. He's got it bad."