Do you want his write-up to just reflect if he was statted out in the DFRPG system, or do you want some suggestions about where he and Halloween Town might fit into the Dresdenverse?
I kind of agree with Sanctaphrax about him being a Fae because it seems more in line with the idea of the NeverNever (in the movie he finds the grove of trees with doors to other holiday worlds - maybe a place like that really exists in the NeverNever and that's how he found them) and a denizen's Demense where he is only able to leak into the mortal realm easily on Halloween.
If you wanted to take it from a non-Fae angle maybe you could make him an old-timey god who took on the role of Jack Skellington as the holiday became less about its Christian roots and became a holiday of ghosts and scares and candy. Or like
a certain holly-jolly Norse God that we know
maybe Jack takes on different roles throughout the year vis a vis Day of the Dead:
https://en.wikipedia.org/wiki/Day_of_the_DeadFor actual stats like Sanctaphrax said high Presence and Performance really seem to make sense maybe in conjunction with high Intimidation and Stealth to represent his scare capabilities. Incite Fear and Emotional Vampire seem to make sense, maybe add in some level of Speed, Cloak of Shadows, and/or Supernatural Sense (Fear).