I find S1 Justin very hard to write, and generally don't care much for S1 fic, particularly long or serious fics. In very short stories, drabbles, and humor, his charaterization, if well done, is more likely to work for me.
I think this particular story does a good job of capturing his combination of toughness and determination coupled with his teenager-y tendency to try to be who Brian is pushing him to be. It makes me think of that scene where he and Daphne go to the tattoo parlor, and Justin is all "I don't do boyfriends." It's like he's being all tough and strong, standing up to Hobbes at school, and his parents, but then trying to be what Brian wants him to be. VERY teenagery. And I think this fic captures that very well.