YA Fantasy Novel, Boy Makes Deal with Demon, goes to a world inside of a monster

[Ashleigh]

I barely remember the details on this one, but when I was in high school back in the late 2000s (I want to say around 2007-08) I read this weird opener to what I think was a series. This kid makes a deal with a demon and I think started to become one along with his friend? Spoilers ahead

He consumed food from this demon world and started to become a demon, but the demon world ended up being inside of a creature's stomach or something and he had to stop his friend from ending that world in order to undo our world because of all the bullying he faced, and it ended with his friend screwing off to somewhere else and a hook for another book. It feels like a fever dream but I remember these distinct details including the demon side character being amazingly smarmy and I'm wondering if anyone else read this trippy book and knows it's title?