You can only hook up with him at the moment. He's a special NPC so he's locked in a relationship with the cheerleader captain. Maybe in future updates, partnered special NPCs could be broken up too just like with partnered regular NPCs
It basically depends on their part in the story. Romance isn't possible (yet) with the quarterback, but you can have a D/s relationship with the RM/RMP since it's in their storyline. Some special NPCs (the rich kid, QuickieBurger owner, etc.), even if they're single, also aren't romanceable to male protagonists because they're straight.