I pretty much explained everything needed to know in the previous idea. If you make a story based on this idea please pm me so I can read it :)
Idea 2: During Battle of the Labyrinth(BotL) When gods fade they choose an heir that will take care of their domains. Pan just like all gods had to choose an heir. When he is visited by Percy, Grover, Annabeth and Tyson he knows that this will be his chance to choose an heir. Although Grover would be the better choice pan knew that Percy would do the best job of taking care of his domain. That Is why he selects Percy son of Poseidon to be the new god of the wild. (After The last Olympian) His powers start to develop but, he has no idea how to do control his new found powers and that is why he asks Artemis for help.
This is my second idea. I will update this as soon as I get an idea