Chris is EXTREMELY nostalgic based on this definition of the word. High School is the greatest example of how Chris longs for the days of his past well after he forgets how uneventful they were. Teenage Chris didn't appreciate High School, it was something he was forced to do in between Pokemon cards and vidya. He read children's fiction instead of socializing or learning, or played Gameboy while his Principal appointed friends engaged in girl talk. Years after the fact, he's obsessed with the idea of High School being this pinnacle experience in his life, and he spends most of his time creating a Lego high school fantasy scenario where he's a popular, attractive, straight, Tomgirl male who has a gaggle of sexy female friends.
Nostalgia is often coupled with idealization. It's easier to say that the past was better when you choose to forget all the bad stuff. It's like how Rockabilly scenesters will tell you that the conformity and traditional values of 1950's culture are superior to our current Liberal world. They're leaving out the racism, homophobia, violent bullying and misogyny that were realities of that era. In Chris' fantasy high school, he depicted a "nerd" who wasn't him getting a swirly in the boy's bathroom. Chris is idealizing that he wasn't a social misfit because someone else was filling that role, and they must have received the terroristic bullying he's familiar with from children's entertainment. The truth is that Chris got picked on specifically, and often, to the point that school officials and other students got involved. Perhaps that's what Chris is so nostalgic for: people rushing to his aid and paying intensely close attention to his feelings.