I like that the story is Fantasy/Mystery, Troy Suza. I haven't seen many (if any) Fantasy Mysteries.

I have three suggestions for the logline:

#1) Change the first part of the logline to: "After a _____ (adjective) teenager with an imaginary friend is caught up in a series of bizarre murders."

#2) Change "he must convince" to "he tries to convince," "he attempts to convince," or “he fights to convince.”

#3) I suggest explaining what that "something" is instead of saying "something they could never imagine." I think "something they could never imagine" is vague. Too much mystery in a logline isn't a good thing. It might cause a producer, production company, etc. to pass on the project.