I am looking for a fan fiction in which Harry gets placed with Severus quite abruptly. It is Dumbledore's ploy to keep him safe. Harry's new name is Tristan. Dumbledore spreads it that his mother died, hence leaving Tristan in Severus's care. Severus attends Order meetings and asks Molly for parenting advice. I read it last year or the year before.
Harry's pretending to be Snape's son, Tristan (who doesn't actually exist), and the son of an old . . . dead . . . girlfriend of Snape's.
And of course it was Dumbledore's idea.
Here is the part where Snape gets parenting advice:
"Was there anything in particular you needed to speak about, Severus?" [Molly Weasley] asked.
"Yes," he said, trying to think of the best way to ask them how to be a parent. Severus snorted into his tea, surprising even himself. How to be a parent, he thought bitterly. How to parent a child that is not mine, who has no respect for me, who never fails to find trouble. The two Weasleys looked at him with worry. No, he was not acting like himself.
"Yes," he repeated. "About Tristan . . ." he said.