Jack, over the course of a weekend, is tasked with watching his little cousin, deal with a bully, and navigate two crushes, of which one is reciprocal.
The cycle of bullying and trauma, from Jack's older brother Tom, to Shane, then to Jack, is explored in a interesting way as it establishes how bullies are created.
Ben greatly helps "King Jack" because Ben is our entry point to understanding Jack. Especially since Ben is the first one to get Jack to open up and allow us to understand his life.
Unlike many coming of age movies, "King Jack" isn't dominated by the goal of dating or sex. It is important, and seeing Jack nervous is adorable. But it isn't the be all to end all.
As noted, Jack isn't the easiest lead to get attached to. While a victim at times, he can also be a instigator, and it could take longer than some want for the tables to turn.