Found: Fiction, teen book, Character named Jonathan

Jonathan lives with his aunt (?) and his mother isn’t present because I believe she’s dead, but he tries his very best to look for his father. There’s this.. “underworld” that Jonathan goes to, filled with monsters. I believe his dad is in trouble or something like that. I read this in middle school and I know there a series, the school only had book 1 and 3 I think? Would really love to find this book. The cover looks red, with gargoyles and there’s a castle in the back? And a metal fence. Title is in either green or yellow.

Darkside by Tom Becker.

>2 MissSquish: THANK YOU SO MUCH!

Touchstone: Darkside

>4 Caramellunacy: Thank you!!