story identification - Trying to find a book about a kid who ends up being summoned as a demon

I´m trying to find a book about a kid who tries a drug, ends up in an out of body experience, and his soul then gets summoned against his will to serve as a demon by a wizard.
At first they think he is a low leveled demon but he tries to escape and fighting back, and they then think he is a high ranking demon instead.... Thus he ends up looking like a huge demon.
I remember the book cover looked like a giant minotaur stepping through a portal (Made in poor CG) but I can´t remember the name of the book or who it was made by.

any help would be appreciated. Many thanks in advance.