Goblins by Royce Buckingham.

Along the lines of Suzanne Collins' Gregor the Overlander, Buckingham creates a macabre, wacky underearth, populated with human guardians, oversized bugs, and horrendous (but pretty funny) Goblins. After PJ (17) and Sam (12) run into an unidentified animal with PJ's dad's "borrowed" police cruiser, they receive an unexpected visit from "animal control." Sam, realizing he is on the cusp of great adventure, follows the two "animal control specialists" into a vast network of tunnels and enormous caverns--the "Underearth." Reluctantly, PJ follows, only to find himself seen as a savior for the overwhelmed guardians, the humans who protect the earth from the threat of Goblin invasion from the Underearth. Lucky for both of them, the Goblins are as inept as the guardians are heroic, and the humor flows as freely as Goblin blood. In the end, all is made right, with the hint of a sequel to come.