Shaun's last name is identified in the DVD extras, I believe in the director/actor commentary. Interestingly, only Shaun has a declared last name; Zach's last name is a mystery. They said that in the scene where Zach goes to talk to the admissions office at CalArts, he tries out several last names, but none of them "took," so they took it out and just left his last name undeclared. So, there's been a billion last names for Zach across various fics.

OK, so now onward to your next Shelter story! :D