Disclaimer: Harry Potter belongs to J.K. Rowling. I make no claims to ownership.
Only semi-compliant to canon.
Ginny looks radiant with the baby in her arms. "Better let the family meet him before they break the doors down," she says wryly.
Harry carefully takes James from her. Outside in the waiting room, the Weasley family crowds around him, and Molly beams with pride as she is handed the baby, cradling the head of dark black hair. Even Andromeda looks quite pleased. The only one who doesn't seem happy about the new baby is a sullen little boy with bright blue hair.
With the family distracted by James, Harry walks over to Teddy and crouches in front of him. "What's wrong, buddy?"
Percy, who is close enough to hear the conversation, discreetly turns away to give them a semblance of privacy.
"Nothing," Teddy mumbles, crossing his arms.
Harry has an inkling he knows what this is about. "Just because James is here doesn't mean we love you any less. You know that, right?"
Teddy bites his lip, looking extremely doubtful.
"We're just really happy to meet him," Harry continues, searching for the correct words as he speaks. "It's been a long nine months, and no one was expecting James to come early. I know your dad was just as happy when you were born."
"But I don't know him!" Teddy cries. "You're my daddy. Not him."
Harry realizes he should have expected this. All Teddy has of Remus and Tonks were pictures; he has no actual concept of their love for him. In contrast, Harry has been there for almost every milestone in Teddy's life. He had been holding Teddy when the boy first said, "Dada," and both he and Andromeda began to weep, to Teddy's confusion. When Harry and Ginny began dating again, she had been the one to teach Teddy how to ride a broom.
"Hold on a second, bud," Harry says, getting to his feet.
Hermione is currently holding James. She's gazing at him with such fondness that Harry knows she and Ron will be starting their own family soon. They're getting close to twenty-five, which is the age Hermione had planned to have her first child.
"Find me after this," Harry tells her as he takes James from her. "We need to talk about something."
Her brows knit, but when he smiles reassuringly at her, her eyes widen and fill with tears. Harry thinks she might have figured out she'll be named the godmother, and as he walks back to Teddy, berates himself for ruining the surprise.
Getting down on Teddy's level again, Harry lets him look at James. Teddy only gives the baby a dark glare.
"There's something I want to ask of you," Harry begins carefully. "It's a big job, but I think you can do it. You'll have to be his older brother, Teddy."
Teddy raises his gaze to him uncertainly. "Brother?"
"Brother," Harry confirms with a smile. "You'll have to teach him and protect him and do all sorts of things. How does that sound, Ted?"
Teddy looks at the baby again. There is a long moment in which he considers the idea before turning back to Harry.
"Can I call you Dad?"
A stab of pain goes through Harry's heart.
"Yes," he says, because even if he wants Teddy to honor the memory of Remus and Tonks, he knows that Remus, at least, would approve of this.
Teddy grins. "Okay, Dad."
AN: I know a lot of people compare Harry and Teddy, but their situations are different. Harry, at fifteen months old, likely had a better understanding of his parents and knew that Sirius was an 'uncle'. Teddy, however, was only a few days old (excuse me while I go sob) and can't really understand the idea.