My answer is going to be a little more complex, but it kind of bridges the two options so bear with me here. I think that Chris does have a tragic case of nostalgia, but... I don't believe that his current obsession with Lego and vidya are part of that. Amongst other things, Chris is a manchild. He's never grown up. There was never a point where he STOPPED playing with Legos. His toys are just a continuation of his childhood, as other posters have said. Indeed, he almost certainly has no concept of time (partly because his cognitive processes are delayed, partly because he's ignorant/just doesn't care, and partly because he doesn't have any job or responsibilities).
However, Chris is a deeply depressed individual, and it comes across increasingly in his few later videos and his equally sparse FB updates. His life sucks, and he wishes for happier times, which (for him) would be high school. Having absorbed so much of American popular media, especially books and TV shows aimed at teenagers and young adults, Chris fetishes high school. He views himself as younger, thinner, more attractive, and (most importantly) smooth with the ladies. I'm not entirely sure his gal pals confirming they weren't friends with him has sunk in. Likely he's still in denial about it. Can't say as I blame him, because (like so much in his life) it is pretty depressing. So yeah, he's definitely nostalgic, but not everything is an expression of that.
The place where nostalgia and his continuing childhood spergatory overlap, however, would be his replica MHS Lego model... which looks nothing like the school, and is apparently built over a Cherokian burial ground, because it's haunted by crude, hedgehog faced abominations, angels, and Chris as a tomgirl (a "trend" of one which Chris didn't adopt until well after Count Graduon, but like I said, Chris likely doesn't have much of a sense of time).
