Think about this for a moment. Think about how hard it must have been for him to say those words.

‘He’s not your son.’ 

Like no no no, Molly, this is my Harry. My kid. James and Lily’s son from his appearance right down to the way he writes the alphabet and protects his enemies. I’ve known the kid literally since he was born. I know what James and Lily wanted for him. They’d want him to know what he’s getting into. They’d want him to know that we trust him. And Lily would skin me alive if I let her son face the “chosen one” scenario without knowing what it means. James and Lily Potter gave their lives as a result of this Prophecy and you’re telling me they would want to keep him in the dark? He is my godson, Molly. I would do anything to keep him safe. I’m the one Harry wrote to nearly every day for months and I know what he needs. I know what happened in the damned graveyard. I know what Harry’s been through and I know what his parents would want us to do. HE’S. NOT. YOUR. SON.

‘He’s not your son,’ said Sirius quietly.

Sirius is canonically the sort of person who’d get increasingly louder and angrier over the course of an argument. But no. Molly wants Harry to be a child. Her child. And all he can think of is  Lily. Her grit. Her principles. The way she’d have laid the truth out before Harry and then taken him out to a Quidditch game or something. 

He never gets to say any of that. There’s Molly’s below-the-belt Azkaban taunt and Sirius just retreats into his guilt about not actually being there for Harry… not being able to protect him last year… not keeping James and Lily safe.