He gets the letter and it's a comfort; it really is. It's just that now since his parents (Mum especially, judging by the handwriting and how quickly the letter came) have set such a firm stance on the matter, he feels obligated to tell Dumbledore. If he's not going to do it, then surely the Headmaster needed to know that, right?

So it's back to Dumbledore's office he goes. He's just about wrapped up his explanation of "Mum said I didn't need to" when quite suddenly his mother is there, a startling force of rage. Dumbledore doesn't seem surprised-- he'd nodded in seeming understanding in response to Harry's words.

The Headmaster still remains relatively calm, honestly. His hands fold in front of him as he looks back at Lily Potter, his voice even as he begins, "Lily, surely you understand. The prophecy does state--"

But anything he does say is probably overshadowed by the startled and confused "Mum?" that escapes Harry's lips upon seeing her.