Surely this must be Harry Potter and the Ripple Effect by aurtrina? A later chapter has a nice summary:
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 ...
Is this The Draco Trilogy? One source describes it thusly:
The Draco Trilogy by Cassandra Claire
Word total 896,762 (Harry and Draco end up in each other's bodies semi-permanently after using a botched Polyjuice potion in class, only no one else knows.)
It has three books with titles starting with "Draco" (the trilogy) and two others that don't:
I believe that the fan fiction you are looking for is “The Heir of Gellert Grindelwald.”
What if an escaped, Ex-Dark Lord stole an abandoned Boy-Who-Lived from the doorstep of his aunt's house and raised him as his own? What will happen when Albus Dumbledore comes across the adopted son of his enemy and teen love? And how will the Wizarding World fare ...